I have to agree with the previous posters; GTA IV is awesome. I have GTA 3 and GTA San Andreas on the PS2, but they never got me completely hooked like GTA IV did ..
Oh, and Uncharted: Drake's Fortune was also an excellent game.
I still have Ratchet & Clank and MGS4 in my "got to play" list, and they're already bought ..
I've also wasted quite a lot of time on "Stardust HD" that's downloadable over PSN. Quite a fun game, and reminded me of the good old Amiga-days