I would say Infinite, but if you're looking for something you can invest time into, go with Defiance. By all means Infinite is an amazing game, but you could easily rent it and beat it and buy Defiance.
Bioshock Infinite. It's one of the best games of the last few years. And no, you don't really need to have played the first two games to understand the story, but you might enjoy Infinite more if you have.
Bioshock Infinite. by far a better game.
BioShock Infinite is better than any game that came out last year, even Spec Ops and TWD.
It depends on what you're looking for, but I'll go ahead and say Bioshock: Infinite (even though I haven't played either...) because I've heard more positive stuff about it. And from what I hear you don't need to have played 1 or 2 in order to understand it, but it's better if you have.