PSP2000 is the best I've used. I got an Ice Silver and modded it for a total of $65, played everything from the 2600 up to PS1 and GBA games near flawlessly. Seriously, with the power of the machine and variety of things it can do, playing all PSP and PS1 games is a great extra, it is the best deal IMO. Plus it has that classic Sony feel.

I've heard great things about the Dingoo though, and the GP systems seem to be nice as well, but for the low cost of the PSP and ability to play Sony games, it's my pick.