GB - The first one, none to be backwards-compatible /with/, so it's ok.
GBC - Adds color. Plays GB cames ok, with a bit of color even.
GBA - Plays GB/GBC games - majorly screws with the color pallettes in GB games, though (like Kirby's Dreamland, for example.) And it drops the IR port. Still, not bad.
GBA SP - The lit screen is good, but the reconfiguration doesn't exactly help things like the e-reader, Kirby's Tilt and Tumble, the gameboy camera, or other things depending on the orientation of the cartridge or proximity to the link port. Still no IR port.
Nintendo DS - I know, not truely a GameBoy, still... It plays GBA games. That's good. But it has no link port, and it's wireless isn't comptible to the GBA wireless adaptor. And it can't even play GB/GBC games at all. Terrible.
There should be a way to make a Nintendo DS+ or something, that can play DS, GBA, GBC, and GB games, with the GameBoy cart in the correct orientation, a GBA linkport in the correct location, /correct/ color palettes for GB games, an IR port, and compatability with both DS wireless and GBA wireless (maybe a GBA PictoChat client to send to the GBA wireless adapter, that could recieve text/img and send text) And maybe ship it with a black casing or something, black is cool.
Just a thought. And a good idea, too.