Best is a toss up between the 3000 and 4000T. The 3000 had scandoubling, SCSI, and was very well made.
The 4000T had SCSI *and* IDE, a nice big roomy case, AGA, and a certain amount of prestige that it commands.
Worst? They all have their faults. The A4000 has battery leakage problems, trouble with early buster models, and Paula-audio death seems fairly common, but otherwise they're really nice machines.
The 500+ was alright, it wasn't Commodores fault that game programmers relied upon certain undocumented features of the Kickstart 1.x ROMs (or worst of all expecting routines to be at fixed addresses).
At the end of the day, all Amigas are bloody great, imho :-)