Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
I got into this game in 2002. It was considered really old back then...and now it's practically ancient. However, this is the only RTS that I continue to play on a weekly basis at least. I've grown tired of 3D RTS games, they never seem to be as good as their 2D counterparts (command and conquer, age of empires).
Many people compare this game to WarCraft III. Many people like StarCraft better than WCIII, and SC continues to get more players on b.net than WCIII.
7 years later, this game is still selling. This is easily the most revolutionary RTS game of all time. If Command and Conquer created the RTS genre as we know it now in 1995 (excluding Dune), StarCraft perfected it. This game gets my A for being the most balanced game I have ever played, and one of the most fun online game I have ever played on a PC (2nd only to Counter-Strike <3)
Don't even get me started on UMS. This and Halo 2 easily have the most creative and amazing custom games I have ever played in my life. A all the way.