I'm a big fan of the Warcraft RTS' and I think WC3 had probably the best single player component of any game I played in the genre. I can't think of any other where the different faction campaigns build on top of each other serially to tell one game's story (Command & Conquer Generals kinda had it, but it wasn't dependent on a mandatory order). The inclusion of slotted, levelable heroes was a big contribution to the story as well.

Unfortunately, it hurt the game's viability as an RTS. There's already so much to do in competitive matches, the hero aspect was the straw that broke the camel's back. It's probably why the multiplayer AI was so notoriously difficult and WC3's biggest legacy is Dota and the MOBA genre, not what it did for its own genre.