...not easy...
1. Turbo Duo - the look and the capability. The Turbo/PCE debuted in 1987 and was still releasing games into 1994 (if not longer. I've never actually been clear on when software stopped being mass produced for the PCE/Turbo).
2. NES - Not the best hardware, but tons of software make this system a second favourite of mine.
3. Xbox - Halo and Halo 2, Burnout 3, EA Sports games (all were superior on the Xbox), Xbox Live (Sony has never even come close to the functionality of Xbox Live), the Xbox Media Center (my Xbox is getting more use now with xbmc than ever before).
4. GBA SP - Not only can I play old NES and SNES games on this extremely well designed portable, but I can also play old arcade classics as well. Oh, and not to mention that the GBA will also emulate Turbografx games. I'll have to try that out at some point. Here's an emulation comparison video of a GBA vs GP2X:
5. SNES - Though I've never owned one, I've logged many, many hours on this baby. Whether it was Street Fighter II, Rock 'n Roll Racing, Zelda, R-Type games, or Star Fox, my thumbs got sore on this machine.
Honourable mention goes to the Atari 2600. It's just classic. I killed Bezerk....killed it.