All three are very good games (4 including MGA, which does rock).
I would say Ridge Racer, as I am a huge fan of the series, but it may not be for everyone. It IS extremely flashy, loaded with content, very fast, and ultra fun. The controls are a bit too easy, but it does make for an extremely fast and fun ride. Plenty of tracks, an awesome soundtrack, outstanding 60 fps visuals, and tons of content. The Novice, Pro, and EX tours aren't particularly difficult, but the MAX tour will kick your ass.
Wipeout has grown on me as of late. The game did not immediately appeal and it took a while before I REALLY got into it, but once it clicks, you'll become addicted. It's a very tough game to master, however, and requires a lot of practice. At first, it doesn't seem particularly loaded with content, but you'll spend a lot of time mastering those 16 tracks (17 with the free download). You need to memorize those bastards and really come to grips with the handling. It's very rewarding and addictive to earn those gold medals as well. The visuals are also EXTREMELY good and very detailed, but the framerate isn't as smooth as RR (30 fps with some slowdown when lots of weapons and vehicles fill the screen).
Twisted Metal is my least favorite of those three, but I still enjoyed it. It doesn't look quite as good as Black (and is only 30 fps), but it's close. It's a solid game, but isn't quite as loaded as the other two...though it does support internet play.
MGA is also recommended, as it is a very lengthy and challenging strategy game. It holds suprisingly close to the Metal Gear series while taking a new spin on everything. It really does play like a turn based Metal Gear.
I assume you already own Lumines, but if you didn't, that's a must (one of the best puzzle games ever made and a blast to play, even if you typically do not enjoy buying puzzle games). Hot Shots Golf is pretty awesome as well.
None of those games suffer as a result of load times either, which is VERY important. For example, Ape Escape and Mercury are both good games...but there is a whole lot of loading quite often, and it really does get in the way. Those are the only two games I've played with bothersome loading, though.
Oh, and one more thing about Wipeout...
I'm not sure if it's just me or what, but when I sleep my PSP during Wipeout (within a race), there is a very good chance that it will crash within 10 seconds of powering back up. You can sleep in the menu all you want, but sleeping during the game will almost always bring a crash. No other games I own have issues with sleep mode.