I disagree on Wonderswan. The Crystal, being over $100 now, isn't a great option unless you have a lot of color games. If what you have most of is B&W games, they are worse on the Wonderswan Color and wasted on the Crystal. The B&W Wonderswan is very cheap, about $10 - $20. I'd say it's much better value than the Crystal if you like B&W Wonderswan games.
The Game Boy Color is nice, but I find the postage stamp-sized screen to be a letdown. I like that the Pocket has a bigger screen. The Super Game Boy 2 is tethered to a home console, unless maybe you have a portable SNES clone, but that's just ridiculous.
Also, the original GBA has a completely horrible screen, it's second only to the Wonderswan Color in darkness and unreflectiveness (actually, the WSC is a bit more reflective). The original SP is okay, sort of blue-ish though. The SP2 looks great until you ask it for motion, I find the response time to be annoyingly slow. It's just slow enough that if you play an original Game Boy (Color) game, the pixels will become misshapen.