Assuming you are talking about 360 games, as you included Halo 3 and ODST there, I'd suggest buying Borderlands.
Prototype is pretty nice too, but it's a casual game, in my opinion.
On the other hand, you will only be able to enjoy Borderlands to the maximum if you play it for a good amount of time. Unless you have that time in your hands, get Prototype.
However, if you are a patient human being, I'd suggest, like Paralyzer did, that you wait for Mass Effect 2 to come out. It will hit the shelves on January 26, in North America, and on January 29, in Europe.