Starcraft > D2. I was pretty addicted to D2 for quite the long time, but I couldn't go back and play it now. If I wanted, I know I could find a friend to go and play starcraft with me right now, it just never gets old.
Starcraft wins, IMO. I was hopelessly addicted to that game; I played it from when I was 9 to about 14/15. Every once in a blue moon I'll play a game or two and realize how good of a game it truly was.