TL;DR
For the standard 433mhz OpLink/Revo/Sparky2 control/telemetry, buy Retevis RHD-771 SMA-M for both the aircraft and ground station, but these are heavy (34g). If you want lighter with perhaps a small loss of range, get Nagoya NA-24 SMA-M (8g).
I get 200m+ on 1.25mw through multiple houses and trees. I would bet I get over 500m in the air with clear line-of-sight. Just to be clear, I would not fly on 1.25mw with OpLink for control without first testing for antenna range or having RTB fully functional.
One factor is to make sure you don't have bad clone RFM22B's. See thread "Bad clone OpLinks (and bad RF on Revos)"
https://forum.librepilot.org/index.php?topic=4197.0Another factor is RF interference from other electronic devices. Generally something that transmits causes problems with something that receives if it is close by. This includes in order from most problem to least:
- RF transmitters, particularly on odd ordered harmonics, for instance, 433 x 3 (3 being a small odd number) is 1299 which is close enough to 1280 to cause potential problems for FPV video receivers on the ground when a 433 transmitter is close by.
- Switching voltage regulators
- Anything with a microprocessor in it, especially if the processor clock (or an odd order harmonic of the clock) is close to your receiver frequency. A 450mhz processor might cause problems with a 433mhz receiver.
Disc capacitors on noisy power supplies and antenna filters (e.g. a low pass filter on 433 to stop it from broadcasting 1299) can help.
The other main factor is antenna / antenna placement.
Don't use a monopole / quarter wave / Sander style. They need a ground plane to work as designed. Use a dipole. Dipoles don't need a ground plane. Most manufacturers have a quarter wave antenna they call 701 and a half wave they call 771. Stay away from 701's because they are quarter wave and they don't get reasonable range without a ground plane. I have tried several 771 style antennas and the Retevis is good, cheap and consistent.
Retevis RHD-771 SMA-M Excellent range. Simply the best I have tested. Others that have better range are higher gain antennas.
Standard end fed / end mounted half wave.
Excellent ground station antenna.
Excellent aircraft antenna, but fairly heavy at 34g. I would still use it on a fixed wing where weight is not a premium.
Pair I bought for $8.92 shipped. Actual seller I bought from.
https://www.ebay.com/itm/2X-SMA-M-Dual-Band-144-430MHz-U-V-Antenna-For-Retevis-RT1-2-3-TYT-MD-380-WOUXUN/311671803647 Single $4.32 shipped. A different seller.
https://www.ebay.com/itm/Retevis-RHD-771-SMA-M-Antenna-Dual-Band-VHF-UHF-for-Retevis-RT3-Walkie-Talkie/112885110333 Three for $11.68 shipped. A different seller. I just ordered these and will post later.
https://www.ebay.com/itm/3x-Retevis-RHD-771-SMA-M-Elite-Antenna-Dual-Band-Radio-144-430-MHz-UHF-VHF-HOT-Y/172827178368 
Center fed OP wire dipole that comes with some OpLinks/Revos Excellent range.
Center fed / center mounted half wave is more difficult to mount.
Excellent ground station antenna. but difficult to mount.
Excellent aircraft antenna, but too flexible without bracing.
Center fed half wave is more difficult to mount, especially with the flexible wire elements.
I probably wouldn't buy one of these, but you get them for free sometimes and they do work well.
This is a great antenna, but is challenging to mount correctly. The antenna wires need to be held, but by something that is RF transparent.
Best mounting has the two wires straight (vertical) with the cable coming off at 90 degrees, forming a T but sideways like this -|
For aircraft use, consider replacing the non-stiff wire with music wire on top and stiff flexi steel cable on bottom.
https://sasquatchlabs.org/product/mmcx-whip-antenna/ (sasquatchlabs.org web store has closed)
Magnetic base mount quarter(?) wave with center load coil with cable Excellent range. Requires a ground plane. Some sellers claim this antenna has 5dBi gain.
Magnetic mount end fed / end mounted quarter wave "stick on".
Excellent ground station antenna.
Not recommended for an aircraft antenna because of required ground plane and stray mag fields.
For testing I used a 425mmx425mm PC case side balanced on a fence post about 1.5m above the ground.
Your car roof would be a good ground plane if it is simple flatish metal.
Marginal without a ground plane. About the same as "cheap wire coil style".
About $1.90 shipped.
https://www.ebay.com/itm/15cm-433MHz-Antenna-5dbi-SMA-Male-RG174-Cable-GSM-GPRS-Magnetic-Base-for-Radio/172749020354 
Center fed dipole with cable #1 Excellent range. Some sellers claim this antenna has 6dBi gain.
Center fed / center mounted half wave is more difficult to mount.
Excellent ground station antenna.
You would want to cut a lot of cable off and put a new end on it for aircraft use.
These antennas work a little better than the OP wire dipole. I suspect that it has a little gain.
This is not the seller I bought from, but they are probably fine.
About $2.30 shipped.
https://www.ebay.com/itm/118inch-433MHZ-GSM-GPRS-SMA-Male-Plug-90-Horn-patch-Antenna-Signal-Amplifier-TB/162956757563 
Center fed dipole with cable #2 Excellent range. I suspect that it has a little gain, more than 0 dBd (dB dipole) / 2.15 dBi (dB isotropic).
Center fed / center mounted half wave is more difficult to mount.
Excellent ground station antenna.
You would want to cut a lot of cable off and put a new end on it for aircraft use.
These antennas work a little better than the OP wire dipole. I suspect that it has a little gain. Comes with a long coax.
This is not the seller I bought from, but they are probably fine.
About $4.00 shipped.
https://www.ebay.com/itm/Antenna-433MHz-3dbi-GSM-GPRS-SMA-male-plug-tentacle-3M-RG174-cable-UHF-VHF-FPV/271456299464 
I haven't tested this apparently shorter version:
https://www.ebay.com/itm/External-Antenna-433Mhz-2dbi-SMA-3m-RG174-cable-for-Mobile-Phone-Car-GSM-Phone/151406066044Nagoya NA-24 Good / excellent range.
I consider this to be the best antenna for a multicopter airborne because it is light (8g).
I would probably use a Retevis for a fixed wing where weight is less of an issue.
Standard end fed / end mounted half wave.
Good aircraft antenna.
Consider the Retevis for ground station.
This is about 4cm longer than most 1/2 wave 433 antennas. It may be because thick antenna wire reduces resonant length, or because the NA-24 might even be a 5/8ths wave antenna.
Much lighter (8g) than the 771 (34g) style antennas so it's good for airborne. Very skinny wire which narrows the tuning (resonant over a smaller frequency range). I was using LP channels 0-31 (430.000-430.128) for this test. It's quite possible that there is a set of channels in setup which better match this antenna's narrow tuning.
About $15.20 shipped.
https://www.ebay.com/itm/New-Genuine-Nagoya-NA-24-SMA-Male-144-430Mhz-2-15dB-10W-Flexible-Antenna/122292796574 
Nagoya 771 (probably authentic) Usable range.
Standard end fed / end mounted half wave.
Usable ground station antenna.
Usable aircraft antenna, but fairly heavy at 33g.
These are almost as good as the Retevis, but there are a lot of clones out there and it is hard to know what you are buying till you test it.
Nagoya 771 (probably a clone) Usable range.
Standard end fed / end mounted half wave.
Usable ground station antenna.
Usable aircraft antenna, but fairly heavy at 33g.
The Nagoya clones I have bought are not quite as good as the authentic Nagoyas which are not quite as good as the Retevis.
About $1.60 shipped.
https://www.ebay.com/itm/Handheld-Dual-Band-144-430MHz-Nagoya-NA771-Antenna-For-Baofeng-UV5R-UV-82/361954810500 
Nagoya NA-774 Range is not very good.
Standard end fed / end mounted half wave.
Functionally good for a ground station antenna, especially where the telescoping and angle knuckle adjustment is desireable.
Too bad, it might have been good for a transmitter antenna.
About $3.40 shipped.
https://www.ebay.com/itm/2017-NA-774-SMA-male-UHF-VHF-Dual-Band-144-430MHz-Antenna-Handheld-For-Radio/282738901714 
Nagoya (etc.) 701 (or possibly a clone) Not for serious distance.
Standard end fed / end mounted quarter wave.
Might be good for ground station if mounted on a ground plane.
25 grams.
About $5.60 shipped.
https://www.ebay.com/itm/Nagoya-NA-701-SMA-M-144-430MHz-Dual-Band-Antenna-for-Yaesu-Vertex-VX-3R-Baofeng/131945254069 
Other brand 701 style antennas Not for serious distance.
Standard end fed / end mounted quarter wave.
Might be good for ground station if mounted on a ground plane.
Some are much lighter (15g) than Nagoya 701 (25g).
About $7.00 shipped.
https://www.ebay.com/itm/RH-701-SMA-Male-Dual-Band-Antenna-for-Kenwood-TH-F6-TH-F6A-TH-F6E-TH-F7-TH-F7A/222602341149 
Nagoya 666 (or possibly a clone) Not for serious distance.
Standard end fed / end mounted quarter wave.
Might be good for ground station if mounted on a ground plane.
About $2.60 shipped.
https://www.ebay.com/itm/Dual-Band-144MHz-430MHz-NA-666-SMA-Male-Radio-Antenna-For-YAESU-NAGOYA/382269055958 
Small, cheap wire coil style (all the small ones that are less than 1/4 wave long are a coil inside a plastic housing) Not for serious distance.
Standard end fed / end mounted stub / rubber duck.
They work, they are light, some even get range approaching useful (small fraction of Retvis range), but I bought a dozen different ones and have no confidence in finding a version that is consistently well made.
The low gain longer ones are just that much more plastic and still have a short coil inside towards the base end. Often the top is empty.
High gain longer ones can actually have two normal antennas stacked vertically.
About $1.00 and up shipped.
https://www.ebay.com/itm/Omnidirectional-Antenna-TX433-JK-11-2-5dBi-Flexible-SMA-J-Male-Connector-433MHz/112837835243 
Sander Style Not for serious distance.
I have never bought one of these, but they look like / measure a quarter wave and thus would require a ground plane for good range.
About $8.30 shipped.
https://www.ebay.com/itm/NEW-ImmersionRC-433MHz-Sander-Whip-Antenna/263220871215 
Stay away from these "paddle style with cable" ones, not good. Not for serious distance.
Long cable, so probably best for ground station if not the trash bin.
Externally end fed (but not really / inside?) / end mounted.
About $2.30 shipped.
https://www.ebay.com/itm/Antenna-SMA-Male-Plug-Connector-Mounting-DAB-Patch-Aerial-433Mhz-2-5dBi-New/371599028425 
Additional InformationSome final advice is test, test, test. Disable a suspect piece of equipment to see if it causes problems. When you find what the cause is, try this or that to see if it helps. Unplug that FPV transmitter to see if your range comes back. After finding that your problem is a switching regulator (BEC) producing RF noise, try adding some small (say 0.1uf or 1.0uf)
disc capacitors to input and output to see if that helps. If the problem is non-RF power supply noise, some larger (say 10uf to 100uf) tantalum capacitors on the power supply might help.
If not mounted on your RC transmitter, mount your dipole away from metal. Up on a plastic pole works well. You can even put your OpLink up the pole with the antenna.
433 dipoles are about 36+cm long. The exact length may be more or less depending on exact frequency, velocity factor, and local effects. Quarter waves are 20+cm long. Note that these lengths include the base which is not part of the radiator.
Don't get antennas that have gain without truly understanding what you are getting into. Simply stated, antenna gain steals signal from one direction (usually vertically) to make it work better in another direction (usually horizontally). Associated with the letters dBi. 2 (2.15) dBi is as low gain (good) as you will find. 3 dBi is OK. 6 dBi and higher should be avoided. 2.15 dBi (dB gain relative to isotropic, same in all directions) is 0.0 dBd (dB gain relative to a dipole).
When buying, make sure to check that you are getting the correct connector sex. I standardize on cables with SMA-F connectors and use SMA-M antennas. SMA-RP is not the same as SMA.
I was using LP channels 0-31 (430.000-430.128) for this test. It's possible that may have affected the results, especially with antennas that are narrowly tuned such as NA-24, if those antennas happen to be tuned elsewhere.
This testing was conducted with the GCS antenna mounted vertically on a fiberglass arrow shaft strapped to a wooden post with no metal close by. Range may be different if the antenna is mounted on your RC Tx and additionally not vertical.
All antennas tested have a male SMA connector. This is either built into the antenna so that it provides both mounting and electrical connection, or is on the end of a cable, in which case mounting is an additional consideration.
Where possible, I listed the seller that I actually bought from, but that was not possible in all cases. Caveat Emptor.
Info: LP uses a polled RF protocol where (effectively) the coordinator sends data out even without ever receiving anything from the non-coordinator. But the non-coordinator will only ever send when it is given permission to by the coordinator.
I specifically wanted to test the transmit performance so I used a strange setup. I used a rig where the aircraft had an OpLink set as Coordinator and set to 1.25mw and the GCS was set to OpLink Receiver and was run at 50mw. I watched the receive LED on the aircraft OpLink as I walked it around, swapping antennas with power always on, but only 1.25mw power. This LED would only flash if the weak 1.25mw aircraft/coordinator signal was received at the GCS to tell the GCS to transmit. The high GCS power guaranteed that if it received a signal that the signal going back to the aircraft would definitely get through.