I love questions like these. MGS4 I have to concur with Chapu on that one, and taking all those photos of the beauties. GOW III and Uncharted 3 were also plenty 'o fun. I would've picked the Mass Effect games, but they're way more fun on pc/laptop.
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Gory violence, good one-liners and top-notch graphics and stories I think are the key reasons for my choices.
Never really played an RPG before it and it was groundbreaking for me. Being a young gamer it probably has had the biggest influence on me that any game has before. So, Fallout 3.
But if we're talking PS3 Exclusives then it has to be Uncharted 2. I never really got into Uncharted 3 and I thought it was over-hyped. The first inFamous game gets a mention too.