For a powerful machine: The A4000. I know some people hate the case, but I love it. I always coveted a Zorro-capable Amiga (I know technically they can all do it with hacks, but still...) and when I finally got my A4000 it was a dream come true!
For a portable machine: The A1200. It can have most of what the A4000 has in a nice portable case.
For pure fun: I have recently acquired a CD32, and it's awesome.
If I had to choose one though, it would be the A4000.
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moto