I voted Genesis, for purely personal reasons. Objectively, I can look at the SNES and see that it has many great games. But subjectively, it just doesn't *feel* right, somehow. If I had to pick one area where the SNES just doesn't cut it for me, it would be music. Again, I know the SNES had some of the great composers (Nobuo Uematsu, Koji Kondo, Yasunori Mitsuda) but the Genesis had these little flashes of brilliance. Yuzo Koshiro's thundering Streets of Rage soundtracks, for example, or Masato Nakamura's work on Sonic 2 (especially that Final Boss theme... is that *dissonance*, Masato Nakamura? Are you flirting with dissonance? You naughty videogame composer you.)
Of course, none of this stands up to any kind of logical argument. It's just that many of my happiest childhood memories are linked with hours spent playing my Megadrive.
On the issue of the Dreamcast, I'm never entirely sure how I feel about it. Commercially it was a flop, and it had a lot of terrible games, but it has a certain draw to it. It seems to have followed the path of the Saturn, in that many of its best games are 2D, and the 3D games often display a kind of awkwardness (or in some cases are just plain bad - *cough*Sonic Adventure*cough*). It also displays a remarkable tenacity - commercial games are still being released for it, and the homebrew scene is still pretty lively.