Easy choice. Mega Drive.
I'll make a list too.
1. Mega Drive (great)
2. Saturn (good)
3. Master System (decent)
4. Game Gear (decent)
5. Mega-CD (decent)
6. Dreamcast (bad)
7. 32X (junk)
Never used the other systems and I never will.
The Mega Drive is one of my favorite consoles. It is only surpassed by the SNES and the PS2 which has infinite amounts of perfect games. Anyway, there are tons of great games for the Mega Drive. Soleil (Crusader of Centy), Light Crusader, Landstalker, Phantasy Star 2-3 (never played #4), Traysia, Wonder Boy in Monster World, etc. The list is can be made very long.
The Saturn has a pretty solid library as well. Magic Knight Rayearth (one of my absolute favorites), Shining the Holy Ark, Shining Wisdom, Linkle Liver Story, etc.
The Master System has a handful of really good games, like Golden Axe Warrior, Golvellius and Wonder Boy III The Dragon's Trap.
The Game Gear is a really interesting system. It really is a shame that it is a portable. Those games would've been a lot more enjoyable on a TV. The Madoh Monogatari series, Lunar Walking School, Magic Knight Rayearth, etc. Many interesting titles.
Mega-CD is a pretty weak system, but features some good titles as well, but nothing really stands out to me. It has a bunch of "OK" titles, like the Lunar games, Popful Mail and my personal favorite: Dungeon Explorer (which is not just okay, but great!).
I don't understand the Dreamcast hype. When it was new, nobody cared except a select few. Those that DID care hated it, and it died almost immediately. Now, many long years after its death, people are loving it for some reason. I've never liked it, weak 3d is not my thing. There are only a handful of games that interest me for it (Climax Landers, Grandia 2, etc). PS2 was the first console that could properly handle 3d, and it did it well.
The 32X is junk. That's all I can say.