Depends on your budget and what you want to do. If you want to do things like Zwift, RGT, Bkool, Rouvy, BigRingVR etc etc then you will (probably*) need a "smart" trainer. This is one where the resistance is controlled by an app and the app also provides some kind of display (eg a virtual world where you are riding wih increased resistance for uphill sections, or your progress through a set of hard/easy interval workouts)
Some of these services use ANT+ to connect to the trainer, some (most?) use Bluetooth. I know BigRingVR is ant+ only, so if going for a smart trainer then connectivity is a feature to consider.
The other option is wheel-on vs direct drive. Wheel on is where you clip your bike into the trainer with the back wheel in place. Direct drive has its own cassette, and you remove the back wheel and mount the bike onto the trainer. Higher end models are direct drive. Also, if you have bikes with different gearing setups (eg an 8 speed and a 10 speed) it's much harder for them to share the trainer with direct drive.
So: Smart vs dumb, wheel on vs direct drive are probably the basic choices. For smart, check the connectivity.
* Some of these things may work with a dumb trainer too, but maybe to a limited extent, I don't know.
Some good reading (refers to smart trainers only):
https://www.dcrainmaker.com/2019/10/the-smart-trainer-recommendations-guide-winter-2019-2020.html