G6 Lite, M3 (NOT the pro version), Supercard (NOT the rumble version).
I have a G6 and I'm happy with it. It seems to have fewer problems with DS games than the others, and it comes with a shell to fit in the Gameboy. It also has about the lowest battery drain of any cart. The main downside is that it's only 512 megs.
The Supercard is supposedly less compatible with GBA games than the G6 or M3, but more compatible with DS homebrew. It's also less expensive.
M3 pro & Supercard rumble will not play most GBA games.