Assasims Creed 2 has to do alot with stealth and blending in with the crowd, but Modern Warfare 2 picks up 5 years after Modern Warfare 1, plus alot of cool weapons and the best multiplayer ever. So if I had to choose Modern Warfare 2 is the best way to go.
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It depends. If you're more of a multiplayer guy, go with Modern Warfare 2. It has a hard-to-follow storyline, frustrating Special Ops, but its multiplayer is a huge step ahead of World at War. World at War somewhat has a better plot, and it has Nazi Zombies. The problem with Zombies is that it gets stale after playing Nacht der Untoten over and over again. You'll have to buy the DLC with Der Riese to truly satisfy your zombie gaming needs. Plus, if you're a PC gamer, it's really fun to make your own custom maps, although I don't know how.
If you like 3rd person then get Just Cause 2 if you like 1st person Modern Warfare but I recommend Modern Warefare because of its great campaign and awesome multiplayer experience