Ooooh... It's so hard to choose. I have Sega Genesis, Nintendo 64, Super Nintendo and NES as favourites :S....
I would have to say Sega Genesis and Nintendo 64 are very closely tied. When I look at Sega Genesis, and all of the time I spent with burning time on Gunstar Heroes, and beating ALL of the available save slots with the Sonic & Knuckles + Sonic 3 attachment... (Yes... I had saved all the saves to 100% plot completion, including beating Doomsday)
Nintendo 64 had Goldeneye and Perfect Dark, Paper Mario, etc, etc, and I love those games just as much as Gunstar Heroes and Sonic and Knuckles. Just too much good gaming in the past.
Nowadays, I'm finding it hard to get into playing games like Gears of War or sports games, as all it is, is just HD cinematics around a bland engine. Yes, there are some diamonds in the hordes of games, but seriously, the ratio of excelling games versus just plain games is getting worse...