Command and Conquer: Red alert 2 + yuri's revenge
Awesome community, got me hooked on competitive RTS'ing. Great fun whether it was 1v1 quick matches or clan games. Official UK rankings was a nice touch as well. Also the game where I had my first run-in with the unnatural abilities of koreans in RTS games!
PS. It's one of the few games that I'd actually leave the theme tune repeating, it truly got you pumped up for war!