They've already been mentioned, but figured I'd add my opinion...
MGS4, Uncharted 2, inFAMOUS are my #1,2,& 3 favorite games of all time, (in that order); so I recommend them the highest. MGS4 tends to be a game that people either love or hate. I think a great majority of the ppl who play it love it, and often consider it the best game they've ever played (as I do); however, I think it just depends on your personality and preference, bc as I said, there are others who don't like it at all. I personally think it's the definition of a masterpiece.
As for the Resistance games, your cousin may not like them, idk bc ppl are different; but, I think Resistance: FoM has a great campaign (besides in terms of character development). I rented it when I first got a ps3; played for an hour or so, then took it back and bought it instead. I really enjoyed it, and the variety of weapons is fantastic. Not to mention the fact that you can play the campaign split-screen coop. I don't get into online multiplayer though, so I can't comment on that.
As for Resistance 2, I personally was disappointed in the campaign. There are huge bosses and some epic moments, but the gameplay and graphics weren't as good as R1 imho. BUT...as BenVTrigger mentioned, the online 8-player co-op in R2 is awesome. At first I didn't think I liked it, but as I played more and started working toward my upgrades, I fell in love. It was the first time I had EVER become interested in a game's online mode. I still consider my $60 purchase of R2 worth it bc of the coop.
Other excellent games: Ratchet and Clank: ToD (much better than ACiT imo), Heavy Rain, LBP (but I think you said you don't like platformers), Motorstorm 1 (awesome, intense gameplay - and I don't usually like racing games; M2 has 4-player split screen, but it took a leap backwards in terms of gameplay imo).
