I found with SC2, if you're willing to take the Multiplayer aspect of it quite "seriously", then you'll get a lot of fun out of it. Otherwise you'll get bored quickly... And it loses it's replayability quite fast imo. But funnily enough, I was going to mention Supreme Commander myself as somebody else did... definitely go for their advice, if you're looking for a decent RTS to get a lot of hours out of.
Very 'epic' feeling to it when you're controlling different armies across the battlefield in a massive conflict spanning an entire map. But it's a different feel from your usual RTS as well... I wouldn't compare it to C&C, and the 'craft series in that respect, as it's scale is just so much bigger.
Depends what you're looking for... as you say, you like Dawn of War and I'd say DoW is like SC2 in a lot of regards. You build a base, you kill the enemy. Simple. Well, maybe not a lot of regards then...