I've been getting into RTS games lately, a genre I've always avoided, and I'm looking for some good competitive RTS games. They need to be optimized for competitive play because I'm one of those annoying people who are willing to spend hours perfecting my strategy and doing weird math to figure out what is the absolutely best way to play a game.

Obviously there is Starcraft but I'm mostly playing on my laptop right now and it can't run SC2. So yeah, all suggestions welcome, extra points if the game runs on old hardware.

:)

Well what have you played? I'll be honest though I don't know many RTSs, just Starcraft, Warcraft, Company of Heroes and Sins of a Solar Empire.

Here are the games I've played:

Company of Heroes - My favourite RTS ever but I can barely run it on my laptop.

Age of Empires 2 - A very good game but it's not particularly balanced and it's not very good for competitive play.

Warcraft 3 - A great game but every time I play it I get absolutely humiliated. I've tried to look at some strategies but I still get destroyed every time. <_<

I didn't play competitively in AoM, so I'm not sure how balanced it would be.

I love RTS games, but I don't ever play online with them so I'll just rattle off some of my favourites:
Age of Empires series
Age of Mythology
Rise of Nations
Rise of Legends
The Lord of the Rings: The Battle for Middle-Earth
Star Wars: Empire at War
Starcraft(the original)
Probably the one that's the closest to a competitive game is Warhammer: Dawn of War 2. Despite what everyone says I personally don't consider it a full RTS, but it does have RTS elements and is a very good tactics based game.

I played Warcraft 3, but didn't really get into it. It was a good game, but it felt like a smaller game compared to some of the other RTS' I've played and especially to the amazing achievement that is Age of Mythology that came out in the same year.
As a relative newcomer you could try some of the Command and Conquer games(I'd recommend Red Alert 2). I've been playing RTS games for over a decade and while Command Conquer games are still fun there's simply no challenge to them.

The Total War games are good RTS games and have a good online and modding community, at least for medieval 2. I know Shogun 2 was built more with multiplayer in mind and while there is a lot of space for optimizing your army and strategy it can be quite imbalanced.
Not sure if you could run it on a slow laptop but the graphics can be turned down a lot and you can choose the scale of the battles, from 150 men in a unit to just 30.
The Lobotomite is correct in that Red Alert 2 is a game worth playing. I have no idea what the competitive scene is like for it but I can't imagine its that large.

Dawn of War is amazing, especially with the expansions. Gets even better with the http://www.youtube.com/watch?v=neXHcn0lV9A mod for the Dark Crusade expansion, though I'm not sure how well that runs on older PCs.