I really like the old SGI computers, the design was amazing. Not sure how much fun those are nowadays even if you happen to be into CAD or 3D modelling. I friend of mine got an old workstation from eBay and it weighed 50 kg and had a huge power supply.

A really interesting one might be Apple Newton, they have been quite powerful for their time. I think a few years ago I read that there is still a vibrant community around it.