What are some RTS games with active online?

I really want to get in to RTS games but I'm not sure where to start since I'm mostly interested in online multiplayer and I can't tell what that's like from reviews. Can anyone recommend some games they play that have active communities?

So far my list of possibilities is limited to SC2 but I'd definitely like to look at some others before making a choice.

Thanks in advance :)

I would start with the likes of Company of Hero's 2, Rome II , Sins of The solar Empire (the latest one), Planetary Annhiliation, I know its early access but its coming on great now and there's an active community there. Wargame- red dragon, you will probally still find life in Dawn of War II as well although I would check on the steam forums though.

Ones to watch for

Grey Goo, by Petroglyph

Stronghold Crusader II

Etherium

Blizkrieg 3

Thanks Feran. I ended up getting Starcraft despite your recommendation but was able to feel happy with my choice because I had other options to compare to so your post was still very helpful and I'll keep an eye on those games as possible future purchases.

P.s. For anyone who reads this with the same question. I didn't realise until after I'd bought it but there is a free version of starcraft available that seems like it's surprisingly close to full featured.

Company of Heroes 1 & 2.