I'd say MGS4 is the best game I've played on PS3. Plus, it comes with an online version that is just as great as the main game (and aonther game in itself). Other than that, Assassin's creed is good as is endwar and tomb raider. Of course there is also GHWT !
1. MGS4-great graphics and and amazing stealth gameplay
2. burnout paradise-loads of free add ons (motorbikes night time and day time cycle)
3. littlebig planet- great game unique style makes it a must have
4. gt5- best driving simulator EVER
5. GTA 4-huge open world great storyline