from my collection... i'd recommend.
Flower
Trash Panic
Wipeout HD
Shatter
Super Sonic Acrobatic Battle cars
Siren is a great game but huge
if you have finished Ratchet and Clank tools then get Quest for Booty

I thought Fat Princess was worth $15. Got more out of it then plenty of full priced games I've played. Best to wait for a patch to fix up annoying glitches if you go down that path.

Exclusive:

Fat Princess
Flower
Ratchet & Clank Quest for Booty
Wipeout HD
Superdust HD

Multi:

Megaman 9
Penny Arcade Adventures
Super Street Fighter II Turbo HD Remix
Bionic Commando

FLOWER. if you're a human being, you need to play it.

Super Puzzle Fighter is also surprisingly addictive. It's the only game my girlfriend truly appreciates.

Valkyria chronicles DLC (If you own and love VC)
Siren Blood Curse
Wipeout HD
Shatter

Super Stardust HD. Maybe Pixeljunk Monsters if you like Tower Defense games, I personally really love that game. But you should definately get Super Stardust HD. Oh, and Everyday shooter is interesting as well.

Zombie Apocalypse!!! too fun with 4 players and depending what mode itll be insane XD

^I tried the demo and it was fun but it doesn't seem to be something I'd play for a long time.

I think I'll try Flower just for the hell of it, and maybe Super Stardust HD

Definitely Super Stardust HD, Flower is great as well but do not forget about flOw, it just got a trophy patch as well.

I'm not sure what the US prices are, so I don't know what you can get for $20.

But, you need to own Quest for Booty, WipEout HD and Flower. In that order. Fat Princess is fun too, but I doubt anyone plays it anymore.