I can recommend Supreme Commander. It's a great RTS with massive scale battles and is currently on sale along with the Forged Alliance expansion.
I also really enjoyed Command & Conquer 3: Tiberium Wars.
Supreme Commander - it is on sale on steam right now. Huge maps, thousands of units per battle, amazing weapons and super weapons, super fun for coop against well programmed ai.
This is honestly one RTS I keep going back to time and time again.
Supreme commander is on Steam for $15 and is a must-buy for any fans of the classic Total Annihilation. http://store.steampowered.com/app/9350/Supreme_Commander/
If anyone is up for some multiplayer this week/end, hit me up!
This is the game that my friends and I always play at LAN parties. It's controls and interface were way before it's time, and we play it all the time. They totally fucked up with SupCom2 though. Planetary Annihilation is sort of a spiritual successor to SupCom, so hopefully it will turn out well.
Have you played (and loved) Total Annihilation? Have you played (and loved) Supreme Commander? Then you'll love Planetary Annihilation :P
^(we do not talk about SupCom2)
Given that it's an old game, I don't think it will be easy to find.... But here are game made with the same building mechanics. Supreme Commander (Steam link: http://store.steampowered.com/app/9350/?snr=1_7_15__13 ) Supreme Commander:Forged Alliance (Steam link: http://store.steampowered.com/app/9420/?snr=1_7_15__13 ) Supreme Commander 2 (no friendly unit crashing) (final Steam ink: http://store.steampowered.com/app/40100/?snr=1_7_15__13 )
All mentioned are created by some of the devs who are working on Planetary Annihilation...