I haven't played Supreme Commander yet, I got it for like 15 bucks at some random store in the mall, is it actually really good?
Yes, it's amazing.
Amazing. Easily one of the best games (regardless of genre) that I have ever played. Like with Yacoby, it's my favourite RTS, and there is a good group of people on here that occassionally play together if you want people to play it with.
I'm disappointed with the lack of a mention of Sins of a Solar Empire so far. It's another one of my favourites.
Man, I think I'm going to have to play one of those when I get home from work.
As am I. It's kind of different from most other games mentioned as all of it takes place in space, but it is an excellent RTS with probably the longest skirmish games I've ever played. Battles lasting over 8 hours are no exception for me, when playing on the really large maps at least.
Personally I don't really like CoH very much, but that is mostly because of the setting. I prefer my RTS games either with swords and axes or with massive plasma cannons. If you like WW2 it is an excellent game though.
Warcraft 3 is also very much worth playing, as is its Frozen Throne expansion. Both the campaign and the skirmish mode have kept me occupied for many, many hours. And to stay with the theme of orcs, elves and humans beating the [censored] out of each other, Dawn of War and its expansions are really good as well. And in addition to swords and axes you have chainsaws, machine guns and lasers. What's not to like about that?
Rome and Medieval 2 Total War are also definitely worth checking out, though neither of them is of course actually an RTS. Both of those also have many excellent mods, my favourite being http://www.youtube.com/watch?v=CHWzW88DLxo, a total conversion project for Medieval 2. I've spent more time on that mod than on the original game itself.