GBA's always been my favorite system. Weird, huh? FFT:A, Pokemon, Warioware Twisted, the series of Mario rereleases, Superstar Saga, the various ports they had from the NES, maaaaaan. And Metroid Fusion (<3333333)
If we're keeping it to home consoles, I'd honestly say the Gamecube. I grew up with the N64, but the second I got a Gamecube I was amazed by the variety and playability of the games.
Honestly, the 3DS might take the title from the GBA.
edit: No, platform cancelling with C4 really blows. Nothing special.