Best: NGPC, hands down. Every non-casino game for the system is worth playing. Even the casino games aren't really "bad" per se; they do what they're supposed to.

Saturn is pretty high. I would say half of its US library is at least somewhat worthwhile. I would even rank Saturn above Dreamcast in this regard.

I'm not sure how I'd rank something like Game Gear. If you look at the system's library alone, it has a very good ratio. But a lot of those games are ports, and while they're mostly "good" ports, they're just redundant if you have other systems. For example, Streets of Rage for GG was done pretty well, but there's just no reason to play it over the Genesis version.

Worst is easily Game Boy Color, along with PS2.