For me it's hard to pick just one, but I'll be the oddball and say the Amiga 500. It's nowhere near as powerful as the 1200/600 or the big box machines- nor does it offer anywhere close to the same volume of features or upgrade paths- but as a solid "all in one" machine it's great to play most games and play around with.
Of course, I'm a little biased as it's also the easiest to move around. Once you've moved an Amiga 2000 and all it's trimmings up three flights of stairs for a show, you really start to appreciate the smaller form factor of the 500 more and more.