Warcraft III, If you have not heard of it, you've been living under a rock.
This game is fantastic. Not just the game as it shipped, but also the game it came to be.
Let me explain why: The good: - THE BEST EDITOR. Seriously, if you like modding, it'll never get better than this. - Thousands of custom game modes to enjoy. You can play Dota here. Nearly all of the game modes are polished. - An active and vibrant community. There are lots of people working on all kinds of projects, everyone will be happy to help you along! - A huge story driven campaign that sets off the events in World of Warcraft.
The bad: - The RTS speed might turn you off. Its rather slow. - The engine cannot handle too many units at the same time, and lag will only hit those who own too many. (I guess it also functions as an auto-balance tool ;) )