With all the little improvements over the years, I chose the GBA SP / SP2 ( should be merged in one category IMO, because the SP2 just made it even better, but not enough of a change to be regarded as such ).
For years the competition and other Gameboys wanna be , had backlit screens ! I remember when the GB came out in 1989 I was in high school and some of my friend had the Atari Lynx and everybody was amazed by the graphics and the fact that it was backlit, people would say it looked just like a TV !
I don't know how many handled revisions Nintendo went trough, and even when the GBA came out in the early 2000's it still wasn't backlit, while the Neo Geo Poket, Game Gear and other handleds had a backlite like from ever !
Of course if you started with the GBA SP you wouldn't think of how awful it was without a backlite, having to carry around a 3rd party external light, in recent years you kind of give it for granted.
And the rechargeable battery, just like a cell phone it was groundbreaking for a portable system, also we give it for granted nowadays, but God knows how much did I spent on AA batteries in the 90's !
The clam shell folding design was also an improvement, that even though it could be arguably less comfortable to hold, it totally annihilated the need for a 3rd party screen protector or case ( and making it look closer to the original GB design again ), so there you go , an unit that didn't need anything else attach to it to work properly.
To be honest I never purchased a GBA SP because at the time I personally installed an Afterburner in my GBA, so I already had a reasonable good front lit system.
But bow when the SP2 came out, even though it was late in the life cycle of the system, and I already had a DS ( phat ) at the time, the screen was so much better than any other GB, and fully compatible with every GB games out there ( don't forget that the DS only can play GBA games, wile the SP can play even GB and GB color games ), so it was a definitive purchase for me, and I always refer to it as the " definitive Gameboy experience " even better then playing GBA games on the DSL.
My second choice is the DSi XL over the DSL, ( I skipped the DSi entirely ) for obvious reasons.
