Honestly, since Ravnica I don't have a favorite color anymore. That block was the most awesome one ever released by Wizards, and its 10 guilds ( =10 possible dual-color combinations) made me enjoy every single color for its unique benefits.
My favorite of the 10 guilds were probably the Orzhov (black & white) - destruction, protection and capitalizing on your opponent's weaknesses by spamming them with kick-ass tokens.
Before that, though, I was mostly a blue player. Cancel, rebound and control used to drive my opponents nuts.
Other than that, I have a red deck from the Lorwyn block. Can't handle players with protective circles, but if it plays according to plan it kills in 4 turns.