Demon's Souls is the best PS3 game to date. If you can beat the first level without breaking things, you'll play the game for about 100 + hrs.
MAG will make every other FPS game obsolete if you stick with it for at least 10 hrs. Absolutely epic.
Dante's Inferno is pretty terrible. I'd say you'd have to really be into God of War style combat (mash buttons, rinse and repeat for the entire game) to take interest in that. If you play and beat Demon's Souls, its nay impossible to play through a game like this.
Borderlands was pretty amazing for about 40 hrs, but leveling different characters after you've completed it with one is a chore. The PvP is pretty mediocre and is really only fun if played through with a friend. The missions start to feel like a chore, especially if you beat the second play through with one character.
Heavy Rain is really story driven, so if you're into that, it might be your cup of tea. I'm sure I'll play through the game, but I don't see how it could compare to my first two picks.