Mass Effect
The Mass Effect games are among the best RPG's ever made. These are without question the cream of the crop among modern console titles. They're extremely polished and beautiful, have an engaging story and have great replay value. (Update: The first Mass Effect was recently released for the PS3 as a PSN download, making the whole series available on the Playstation 3 console. Previously the first Mass Effect game was a Xbox 360 exclusive title)
Fallout
Fallout 3 and Fallout: New Vegas are great open-world games. The only real complaint I have about these games are how unstable they are. Fallout: New Vegas made vast improvements in stability, but Fallout 3 was horrendously unstable and buggy. That being said these games are so good that I managed to overlook these faults. You can spend hours upon hours playing these titles.
Metal Gear
While I'm reluctant to include previous gen console titles in this list the fairly recent release of Metal Gear Solid HD Collection makes the entire game series available on the PS3, including the first PSOne (PSX) Metal Gear Solid game as a PSN download. The graphics of the first game obviously doesn't live up to the standard of current titles, not by a long shot, but the story alone makes it one of the best I've ever played. The HD Collection also makes the Playstation 2 games available in HD, while still not as impressive as Metal Gear Solid 4 (visually) the story alone makes them excellent value. These games are so good they rival Fallout and Mass Effect without breaking a sweat. I have my doubts about the upcoming Metal Gear Solid Rising because Solid Snake has been replaced by Raiden as the protagonist, but I hope it'll be a good game.
Skyrim
Skyrim is simply an awesome game. Much like the Fallout titles it's an open-world game that allows you to go anywhere you feel like going and do the missions you feel like doing. This game is all about exploring and adventure, but unlike Fallout the world has an roughly estimated 12th century setting with swordplay, magic, dragons and a GREAT story. I don't recommend Oblivion though because the leveling system is, in my opinion, fundamentally broken. I didn't even finish the game because of this. A lot of people would disagree, but I would ask these people to consider all the hoops you have to jump through to get a properly leveled character. It sort of ruins the experience when you have to stand and take a beating from a mudcrab for an hour just to level up your blocking skill. Oblivion is still a good game, with plenty of exploration and adventure, but it doesn't hold a candle to Skyrim.
Dead Space
I really enjoy survival horror games and I'm a big fan of the Resident Evil series. As such I am morally obligated to recommend the Dead Space trilogy. When you play these games you turn off the lights, sit back and just enjoy the horror. These are seriously creepy games with a great story line.