I used different GBAs depending on my mood.
Modded original is the best all around.
SP2 is my favorite system to take on the go due to the excellent screen and clamshell design.
I like playing RPGs on the DS Lite. It has the best screen, but I'm not a fan of the buttons for action games. That said, the DS Lite is probably the best handheld ever when you add in the DS game library.
Revo K101 clone is the most comfortable system with a decent sized dpad and buttons, but the screen isn't quite the right resolution so text looks a little off. I love using it with action games.