Seriously, am I the only one who plays Sins of a Solar Empire on VGChartz.

Sins of a Solar Empire
Sins of a Solar Empire
Sins of a Solar Empire

With a name like "Sins of a Solar Empire" it has to be good.

Age of Mythology and the Titans expansion
The Lord of the Rings, the Battle for Middle-Earth II and its expansion
Supreme Commander and its expansion

These are my top three favorite RTS of all time, and I'm a heavy RTS gamer.

Of course, I have yet to play Sins of a Solar Empire. :)

Dawn of War - Try Company of Heroes (same developer)
Rise of Legends - Try Age of Empires 3. Or the original Rise of Nations if you never played that.
Stronghold 2 (never played) - Maybe Medieval: Total War 2. This one is sort of a stretch I think.
Genesis Rising (never played) - Try Homeworld 2 and Sins of a Solar Empire.
Lords of Magic - Maybe Heroes of Might and Magic 5 (never played).

Most of these should have demos readily available to help you make your choice.

S.T.A.R.C.R.A.F.T.

Age of Mythologies is great for multiplayer, especially teams. That is my favorite for LAN parties.

Sins of a Solar Empire, which I may have mentioned, is a little slower paced. It's really good for Single Player where you don't want smaller missions and prefer one longer campaign.

Dawn of War

Star Craft

Age of Empires (1 and 2, I don't really dig three because I like free-play)

Total War is a great franchise. I have Shogun, and Medieval 1. Both are quite time consuming and fun. I don't know anything about Rome, or Medieval 2... anyone have opinions on those two?

Yeah don't forget about your standard RTS. Of course starcraft. Then Warcraft 3. Then (if you want to dig) Play the original Command and Conquer, probably out dated, but a good game for its time.

EDIT: HOW COULD I FORGET TO MENTION CIVILIZATION?!!?!?! /feels sad for not remembering it...

Total War series, C&C and Europa Unerversalis are all good strategy games, personally i only play strategy games on PC, console controls are really awkward for them