Well, if you want to buy the best JK you can get, look for a 2012+ model, as the ones prior to 2012 have the much less desirable 3.8 minivan engine, which has a host of problems and it severely lacks power. The 3.6 engine in the 2012+ JKs is night and day over the older 3.8. Drive them back-to-back and you'll see what I mean. In addition, 2011+ got the much more modern and nicer interior.
You'll also want to very, very carefully inspect the underside of the vehicles you look at for rust. If in doubt, post photos here and myself and others will be happy to offer opinions / advice.