It's tough to choose, but probably the PSP, followed by the 360.
Both have quality games and ones that I enjoy. The Wii has some great games, and I've enjoyed them a lot. But from perspective purchases alone, I could buy a bajillion 360 games, and only a couple Wii games. Same for the DS and PSP. With the PSP, the options seem endless. With the DS, I feel like I'm forced to wait for the occasional quality title to trickle out, and when you look at a shelf of DS games, it's loaded with so much crap you almost become overwhelmed.
Out of the current library, I already own all the DS games I feel like owning, and it's much more than the average DS buyer owns anyways.