Amiga 4000 for me, definitely. It's the only way to have the compatibility of AGA and the expandability of the big box Amigas. The 2000 and 3000 were both very good, but the 3000 used ZIP RAMS and had limited space. The 2000 was great but only Zorro II. The 4000 has its faults but overall it's the most versatile, and it still surprises me some of the things it can do when it's pumped up enough.
Still have a soft spot for the A1200 though for a quick WHDLoad game, and the A600 is just cute.
