My favourite has to be the A3000. Definitely the Mercedes of Amigas. Sure it was cramped inside, but it was so freaking awesome. If only they had made the A3000+ as the next A4000 (if you see what I mean)
I'd love to get my hands on one.
The best would have to be the A500, the only machine to really deliver amazing performance for a low price (The A1200 was behind the curve when it hit). For many people this was *the* Amiga.
Also another vote here for the A2000 being the biggest ugliest monstrosity of the Amiga Line.