go with a 96 or 99 depending on which body style you like. the 00 and 01 have ****ty engine heads, and anything too old is well... old, sometimes rusty, and could be in need of an overhaul.
many people will tell you to get years 96-99, 92-95 would be ok if you cant find anything else. pre 92 you start dealing with two-piece axles and other crap..
think of it like this. each year, jeep released a more refined version of the previous year (there are very few exceptions, like the 00 and 01 engine heads), so logically you should go with the latest.
this is a few of things you dont want to see in a jeep you're checking out:
HP Dana 30 vacuum disconnect (search google for pictures to identify)
Dana 35 rear axle
4 cylinder motor (2.5L)
cant think of anything else, im really tired. other poeple will post stuf you should look out for too/
96 XJ, geared, 8.8, 5", 33s, dash