Favorite: Easy, Turbo Grafx-16. I love those jack-of-all-trades systems, plus they seem to have gotten more out of their CD attachment than any other system did. Sega Master System also has that jack-of-all-trades quality I love.
Least Favorite? Well, the CD-i comes instantly to mind but they really hardly marketed that as just a game system, see the VCDs out there with the CD-i logo. this may surprise some here, but the system that gets the least play in my house: The Super NES. Yes, I love the RPGs on it, but somehow every other game that came out on it that also came out on the Genesis seems to be more fun with the Genny controller. Plus, the SNES doesn't have Landstalker or Phantasy Star... or the Shining Series for that matter. I guess I just can't get into the regular SNES fare.
Don't get me wrong, it's a great system. I just don't play it.
...CD-i blows.
-AG