Definitely the SNES for me. The N64 was alright, but suffered from a lack of RPGs (which ultimately led me to buy a PSone). Aside from Arcadia, Zelda and Symphonia, GC has been an absolute disaster for me. Very little worth playing, and they ruined Starfox. Twice.
The DS, however, is almost approaching SNES-like status for me. Mainly thanks to Sonic Rush and the general "fun-ness" of games like Pac-Pix and Wario Wario which always raise a smile, especially when showing other people. Once Children of Mana, FFIII, Zelda, etc hits the machine, it'll be a tough call between the two. Kaitou Ruso looks absolutely blinding too! I pray, though, that Konami brings Twinbee to our Dual-screened friend.