PSP for real yo. I've had mine for 3 1/2 years and I still play it every day. There are several important emulators people have not yet mentioned: Playstation (yes, it's fullspeed), Sega CD, MSX, Neo Geo. Also there's decent ports of Dosbox, Amiga emulators, and a bunch of other old computers.
Also, the GBA emulation is
almost perfect. There are only 2 games that I know of that don't work on it: Sabrewulf and the Secret of Mana game. Everything else I have thrown at it, it works beautifully fullspeed. Even the 3d mode7 games which you can't say the same about the SNES emulator.
Plus, the screen is bigger/more beautiful, and the system won't give your hands cramps to play it.
Good retail games I recommend as well: Gripshift (it's kinda like Trackmania), Mega Man Powered up, N+, Frantix, World Rally Championship.
I'm sure the DS has some good games as well, I'd like to get one eventually. But from a pure emulation standpoint PSP is by far the way to go.
Also don't get a PSP-3000. They are definitely not hacked yet, plus a lot of people have complained of weird interlacing issues with the screen which is pretty messed up since it's an LCD.