Dawn of War is RTS game only for Windows :D 
This game is so popular among RTS fanbase, they've movie based on this game as well.
The movie is collectors item and it's worth every penny.

The story of this game is stellar. It's about the time in future, when humans will become intergalactic and come across many malevolent entities or races with whom they've to make wars in order to survive. These extra bionic humans are called Space Marines and their wars are interplanetory. All those vortex, warps, comes into life in the game.  Basically, the game is about using logic and leading your troops to victory. There are other game genre with similar concept or story baseline but I rather prefer to play it on isometric viewfield where I could plan and see everything at a glance. The reason me being RTS fan, that is it :D 

I've played this game over and over again. It's so much fun.

Here is a kick-as-s spoiler :D