Originally my response was based on "if" the Amiga had never existed. But now that the question has opened up to alternatives to the Amiga... I'd still say NeXT/OpenStep, but I'd add the Amiga inspired BeOS. I loved 4.5 and am dying to try 5.0 (it's just ashame my hardware isn't supported) , it's a shame that Palm didn't ressurect it. I'm not too sure about Zeta, is it scam or a legitimately licensed continuitation??? I'm looking forward to Haiku though. I must say that if I were going to use any "PC" OS as an alternative to Windows, Hiaku would be it. Linux is just to cumbersome for my tastes.
I probably also would have checked out the Atari ST family of computers. Unlike NeXT/OpenStep, which I've used work-a-likes of (LiteStep on Windows & WindowMaker on Linux), I have absolutely no exposure to Atari PCs. But I supose my fascination for the Amiga would have translated well to the Atari.
