Looking for decent RTS titles on steam

Sins of a Solar Empire: Rebellion

Spellforce

Hegemony Gold, the total war games (rome in particular)

There are a ton:

Supreme Commander + Forged Alliance.

Total War series (Hybrid of TBS and RTS)

Warhammer Dawn of War series (1 and 2)

RUSE

Wargame: European Escalation

Men of War series

Star Wars: Empire at War

Earth 2160

Hegemony

Crusader Kings 2

Command and Conquer Red Alert 1, 2, and maybe 3.

Company of Heroes

Airmech

Besides the ones that have already been mentioned (SUP COM 2!!!):

AI war Fleet Command (if you're looking for some really challenging AI)

Company of Heroes Series

The WarHammer Dawn of War Series ($20 for the complete collection, was $10 on flash sale, it's totally worth it IMHO)

I own all of these myself plus a few others mentioned above, RTS is probably my favorite Genre to play in

Age of Empires III is ok too.

Cossacks & Cossacks II

THe sheer fact that Total Annihilation hasnt been mentioned so far makes me sick.

Achron is a cool game that involves time travel.

I recommend Company of Heroes, the Dawn of War Series and the Total War Series

what wasn't mentioned and comes to my mind:
World In Conflict
Kohan serie
Setlers 7

If you don't have it already Tropico 4 is on the today's deals for 80% off. It has a decent Metacritic score.

C&C Tiberium wars!

Dawn of War 1 Series

Ive played the base game and the 3 addons around 1500h. Awesome shit. After the years the multiplayer is quite dead but the base game and winter assault have really cool campaigns!