Metal Gear Solid 4. I mean... duh? Anyone who owns a PS3 and doesn't own MGS4 is a disgrace to gamers. No, a disgrace to mankind. Nope. A disgrace to all intelligent beings that have ever existed!
Okay, just kidding ^^ Still my personal favorite of the exclusives, but only worth it if you're into the MGS universe and if you've already played the previous entries.
What I'd definitely recommend to anyone with a PS3 is the Uncharted series. Uncharted 1 and 2 are pretty much independent, with 2 being vastly improved in many areas.
And the Ratchet and Clank games are pretty awesome and original.