This is an easy but great question. I'd have to say my top three are... Call of Duty 4: Modern Warfare GTA IV Assassin's Creed Now although I found GTA IV to have some major flaws with missing content I still feel that it is one of the greatest multiplayer games to be created. Also I very disappointed with Assassin's Creed's replay-ability, once you complete it that it...but boy do you have fun on the way. I guess I mainly put it in my top 3 because of it's sheer beauty. The realism and awe you get from free running around roof tops is incredible!
I havn't played Assasins creed in a while now. I think I'm going to try it before saying what are you thinking putting it into your top 3 because I have forgoten most of it. Also they should add a mutiplayer mode like freerun or something where you and some mates can just mess about in the world. That would be awsome. I think the best multiplayer (Competative) must be COD 4 but for messing around with mates It has to be GTA 4.
Favourite game... Either Little Big Adventure or Monkey Island. I haven't found any newer games that are so great. I know these are old now but I still love them.