Which of these 5 games should i buy?

I recommend either AC2 or you wait for Mass Effect 2

You should try & wait for ME2. It won't disappoint :D

Out of those games you mentioned I'd personally wait until Mass Effect 2.

out of all those games i have 3 of them and me2 on preorder and i'll say this

if you want a game now get ac2 UNLESS you have 2 or more friends with borderlands

but if you can wait i'd go with mass effect 2

ME2, it will be worth the wait and will be better than any other game you listed.

You should probally get the games in this list if you could get them all

ME2

AC2

L4D2

Batman AA

Borderlands

Assassins Creed 2

I think you should go with AC2 or ME2. Batman AA was fantastic too.

If you dont mind racing games then i would say get forza motorsport 3 , its a long games with alot of cars to keep you playing for a long time, if you dont liek racing gmaes then i would say left 4 dead 2 or assassins creed 2.