This depends on what other games you previously enjoyed. Obivously there's GTA IV, but other standouts include: Uncharted, Resistance:FOM, Ninja Gaiden Sigma, Motorstorm, Burnout Paradise, Call of Duty 4, Ratchet and Clank, Oblivion, Guitar Hero, Rock Band. Be mindful of what game genres you already like/dislike. ONCE in a while I've gone out of my usual and found a good RPG or FPS (not really a big fan of these) but I've been burned listening to recommendations and ignoring that I just don't like certain types of games no matter how great they are in the genre.
Thankfully there are a ton of demos available to download (including most of what I listed above). There are some quality titles, just keep reading around to find what you like.