Starcraft, simply no competition.

Starcraft by a long shot. There are, no doubt, some heavy contenders though... for second.

there is only one sensible answer for this question: Starcraft

In terms of build quality

1. Starcraft
2. Red Alert 2
3. Warcraft 3
4. Age of Empires 2
5. C&C TiberiuM Dawn

In terms of nostalgia
1. C&C TiberiuM Dawn
2. Starcraft
3. Red Alert 1
4. Warcraft 2
5. Red Alert 2

NOTE: I have not played SupCom yet...

i haven't played many RTS... My number one would be Warhammer 40k Dawn of War

Red Alert 2

StarCraft is the ultimate I think. Anyone can play but there is plenty of room for improvement beyond tech trees.

WarCraft III FTW!!!

Oh, I forgot Dawn Of War.

It's an amazing RTS, especially if you know a little about the Warhammer 40k lore.

StarCraft.