How about a breakdown by decade?
For me:
70's - Atari 2600 - all I had!
80's - Commodore 64/Amiga WB 1.3
90's - Amiga 2.x, DOS, Windows 98 Second Edition
00's - Windows 2000 PRO - It just Rocked, Linux, Windows XP
I've used a lot more.. but these are ones I like to still play around with...
Cheers!