It depends on what your needs are. My recommendations:
Cobra, it is good for both ground and aerial attack. It is also decently fast, faster than the Scout, and it is a good choice if you don't like the plane controls.
Warhawk, it is very fast, making it very useful for getting heists to the criminal base. The missiles are a bonus, they are not good for ground attack but they do relatively okay against aerial vehicles.
Nighthawk, think of it as a combination of the Warhawk and the Falcon. If you think having missiles and bombs is a good idea, buy this. The cloak is useful, too. The Nighthawk has a slower top speed and acceleration than the Warhawk, though.
Don't bother with these:
Plane, it is slower than the default Camaro. If you need to transport a large number of players, you are better off using something like The Infinity.
Buzzard, the missiles take much longer to lock on than the Cobra. Plus, it is slower than the Scout. The extra money for the Cobra is worth it.
Falcon, it's not bad, but there are better options. It's decently fast and the bombs are pretty good, though.
If you play as prisoner, look for the mining area, pick up a pickaxe, head to the cracks in the wall, and start breaking it. This is an easy, reliable way to escape that does not require you to search through garbage cans.
If you don't care about XP and you don't have the VIP gamepass, focus on mini heists instead of main heists to get cash.
Buy aerial vehicles first. They are easier to handle, usually very fast, and sometimes weaponized. I would recommend purchasing the Scout first. If you can get richer, good vehicles would be the Warhawk and Cobra.