Hot Shots Golf really is AAA. So long as you have even a mild tolerance for golf as a sport, you'll have great fun.
MLB is the better of the two PSP baseball games. The commentary is fluid and it's very sharp graphically, although the cutaways and warm ups were removed. After a while, I didn't mind the lack of franchise so much. It's also the only PSP baseball game with online play.
Untold Legends is very addictive, especially if you're into hack and slash games where you're constantly outfitting your character and upgrading their stats. I rented it on a whim from GameFly, logged something like 40 hours, and eventually just up and bought it
This is one of those games that has been selling very well, mainly on word of mouth and test drives. If you have friends with PSP's, and can convince them to buy the game, I'd elevate this to MUST BUY status. The ad hoc mode is insane great fun.
NBA Street Showdown is really good too. The controls don't work necessarily the same as the console game's, because there's a delay, but you forget about it real fast. I had loads of fun with it. There's about 30 NBA stars, about two dozen legends, and some crazy custom players. You said you liked NFL Street... having seen NFL Street, I can honestly say you'll probably LOVE NBA Street, because it's more polished than NFL Street (better visuals, more stuff to do, etc).
I'll add Darkstalkers and Dead To Rights to the list, with caveats. If you're a fan of Capcom's 2D fighting games and don't mind 10-15 second loads between matches, then Darkstalkers is more than good. Some people have complained that the d-pad is stiff and unresponsive. I can't disagree, although I've acclimated to it. As for Dead to Rights, it's a mindless shoot 'em up -- but still twice as good as the recent DTR2 that was on PS2. I'm so-so on it, but a friend of mine is totally loving it right now... just snapping necks and shotgunning thugs over and over. Perhaps try to rent these two beforehand.
