The first generation(s) of Gameboy is my fave because there were tons of great games.
I am not considering TE or Nomad in this discussion because I associate the libraries with consoles themselves. TE and Nomad are in a class by themselves.
Lynx had only a handful of good titles, but I really liked it. I wanted to like Game Gear more, and it should be second place, but I got burned on a lot of the games I bought for it.
These days, I'm most excited about Game Gear because there are many games I have yet to try. I picked up the Golden Axe RPG a few years ago, for example, but I still haven't had a chance to play it.
Neo Geo pocket -- sorry, the library of games don't interest me too much. It was fun to play around with it, but it didn't have long-term appeal for me.
GBA -- I only bought a handful of games, but I wanted an illuminated screen for the new century. When I get one of the newer versions, I might try exploring the library some more.