Definitely Gran Turismo 5. I'm easily 200+ hours into it and still have plenty left to do. By far the best driving experience available on consoles. Some clunky menus, but everything else about it is top notch. And at this point, you benefit from all of the big updates, patches, and DLC that's been added.
You already got off to a good start with Uncharted, so I'd say your next step is to start digging into some of the incredible exclusive PSN offerings: Journey, Sound Shapes, Everyday Shooter, Dyad, etc.
Also, if you're into fighting games, the community's much stronger on PS3 than on 360.
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