Myself and some friends played through a ton of Sega CD games two summers ago. After that, I determined that it had the best good-to-bad game ratio of any system. The only truly bad game is Marko's Magical Football, and then there's a small handful of average games made by European developers (like Three Ninjas Kick Back). Of course, one of the main genres pursued by the Sega CD are FMV games. You have to appreciate games based on the Dragon's Lair, Make My Video, rail shooter, and Night Trap formulas to really enjoy it.
Since then, I've played a LOT more video games. I think the two systems that come closest to the good-bad game ratio are the Saturn and the Turbo CD. Those systems are absolutely chocked full of good stuff. I haven't played the NeoGeo Pocket Color or the Vectrex, but I think those two have potential to be the best.
As far as the worst goes, I can't imagine anyone picking a system outside the bottom tier. I was thinking the Game.com, but Game.com has a 1:3 good-to-bad game ratio when you actually look at the library. I'm not quite sure, because we often don't play all the bad games on old video game systems. Out of the top tier systems, I think it's a very very safe assumption that the Playstation has the absolute worst ratio. So many games were released, and most of them were utter crap. At least by the time the Playstation 2 came out, even the worst efforts were generic clones that managed to be playable.