I've played all three of these so I'll give you a short run down of each one.
Call od Duty 4: MW- Without a doubt one of the best FPS ever made. Looks, great plays great. Suffers from a short single player mode, but the Multiplayer easily makes up for it.
Call od Duty: WaW- In it's self is a ok. Tryarch screwed some stuff up as far as enemy AI goes, but it was alright, The Multiplayer was ok, For me the whole WWII thing is just played out so I lost intrest in this game really quickly, but if you are a fan of WWII shooters look into this.
Far Cry 2- Dident have this game very long, not for it being bad I picked it up shortly before WaW was released, and when it was I was short on cash so I used this as a trade in. What I did play though It is a solid game, and looks amazing, the problem I hear most is though, is that it gets repetitive, and the story starts to drag about 1/2 way through the game. I never went online with this, so can't tell you about the multiplayer.
Hope this helps. My best recomendation would be Call of Duty:4, but choose whatever suites yuo best.