Probably the best ratio is the Vectrex. I have most of the library, and everything I've played is at least decent. The runner up would have to be the Neo Geo Pocket Color. If it weren't for the gambling games, it would have tied with the Vectrex. The Neo Geo and Neo Geo CD are a close third.

As for worst, I'd have to say that just about any dominant system could qualify for this, from the 2600 to the NES to the Playstation. We tend not to notice, since we tend to only think about the good stuff, but even in their heyday, the libraries of these systems were mostly made up of half-assed crap. For those who don't believe me, try watching Chrontendo. For the most part, out of the 16 or so games an episode, only 1 or 2 are noteworthy and at least half are just shit. I still love the thing, but that's a terrible good to bad game ratio.