Please help a new PS3 owner to choose a game

[Juvenile]

If you have never played assassins creed i would say start with the 1st one.
It will get you into the story and then once you start with number 2 or brotherhood or revelations it will just be 100 X more awesome.

The 1st game was nice but from number 2 onward it was just amazing.

I would also say try the Mass Effect series, i just completed ME3 this weekend and really want to see what happens in the game if i change a couple of my choices.

Regarding Assassin's Creed series - until now.

Assassin's Creed (The Introduction)
Assassin's Creed 2 (Better, more engaging combat, story continues)
Assassin's Creed: Brotherhood (Even better combat and more engaging, story continues)
Assassin's Creed: Revelations (NYP by me, sorry)
Assassin's Creed 3 (NYP by me either, sorry)

Definitely start with the first one. You wouldn't start reading a book from a random page, now would you?