1. Call of Duty Modern Warfare 3. The absolutely brilliant campaign will get you started, and the perfect cooperative and competitive multiplayer experience will have you coming back for more. Highly recommended. (Can't wait for Call of Duty: Ghosts)
2. Call of Duty 4 Modern Warfare. I'll never forget watching the sun rise outside while I was playing it. The modern theme shift was stunning.
3. Call of Duty 1. Like Medal of Honor, but done much better. A fantastic start and a great game.
4. Call of Duty 2. Improves on Call of Duty 1 in every aspect, featuring great level design, impeccable gameplay, impressive atmosphere, and I really loved the tank battles in the campaign.