Has to be Real Time Strategy for me. Standout games include:
Dune 2 - My first RTS and one of the very first games I played. I played the campaign so many times, even though the only unique weapons were a tank and the superweapons.
Command & Conquer - The use of FMV for the mission briefings and cutscenes blew me away when I first saw them. I still play it from time to time even now.
Total Annihilation - The game that showed me that bigger really is better. Bigger maps, bigger unit counts, bigger battles and a bigger soundtrack thanks to its full orchestral score made it my standout RTS game. I'm really looking forward to getting stuck into Planetary Annihilation, as it looks a lot like TA... But bigger! (and with weaponised asteroids

).