they are all different, the old ones are the cj's. very cool, and fun, high maintenance, will require work. 87-95 was yj. 87-90 were basically cj's with new suspension and body, and a lot of emissions. i'd steer away from those. 91 -95 were great jeeps, i'd recommend 4.0 motor, and they're cheaper now, cuz everyone is buying the newer ones. i prefer yj, i'm a mechanic and i'd love to own a cj, but i don't have enough time to keep one up, i'm building a yj now.
97 on up made big changes, suspensions are much improved, coils instead of leaf springs. ride smoother, actually perform better off road, but more car like. every change makes them further from the real jeep.
newest wranglers look awesome, but just another step closer to domesticated.
they're all a different creature, thats where personal preference comes in.
hope thats helpful