I know I probably said Dreamcast earlier in this thread, but to be honest by this point I can't really decide. Mega Drive/Genesis is a 16-bit powerhouse with a large library of fantastic games. Saturn is a tough nut to crack but once you do a little research you find some of SEGA's greatest games including top of the line 2D games (shame SEGA of America didn't push this aspect) and some surprising 3D games (NiGHTS, Panzer Dragoon).
Dreamcast was and is the first console to have 3D graphics with nothing holding the gameplay back, character models represented exactly what they were meant to be representing (no blocking blobs of polygons) and environments were immersive and recognizable (no fog, minimal pop-up). If the Genesis nailed 2D gaming and the Saturn perfected 2D, the Dreamcast nailed 3D gaming in a way that the Saturn never could... but looking back I don't think the Saturn ever was meant to. I blame the aspirations of SEGA's executives who tried to make the Saturn into a machine that it really was not.
Basically:
Master System: serviceable 2D games with some gems, but most were held back by hardware limitations
MD/Genesis: nailed 2D gaming, made attempts at 3D gaming with expansions but largely failed at this
Saturn: perfected 2D gaming, attempted 3D gaming with some successes but not many
Dreamcast: nailed 3D gaming and carried on perfecting 2D gaming, however 2D was never a focus
The SMS is the only SEGA console that I can really point to as being less than great, while the other three all shine in their own ways.