For years, it was the Command and Conquer Series.
Red Alert, Yuris Revenge, Generals. Played them all online. At one time I was ranked in the top 50 westwood players, on Red Alert.
A friend of mine recently turned me onto Delta Force.
I have just finished all the levels of "Black Hawk Down", and currently am hooked on playing Black Hawk Down online.
I now check the forums, respond, go play a few games of Delta Force, then come back and check the forums. Well that's my daytime schedule most weekdays. LOL
Later
Dave (A.K.A. Homebrew)