all of them.
i plan on buying (or have already bought) every game you have listed. good choices, but one is not enough.
though if you are truly limited i would say MW2 or bioshock 2. MW2 because the first was awesome and this si bound to be awesome, as well as haveing an unlimeted ammount of replay value thru the awesome online mode. Bioshock, because the first was amazing, my favourite of this generation.