I've played Warzone, Zero-K and SC2. I think Zero-K is the better of the three. Warzone's huge tech tree is overwhelming and I believe it doesn't really add to the game, because it's likely that only a few tech paths are actually viable if one talks competitive play. SC2 is pretty and all, but suffers from hard-to-solve balance problems. Zero-K has solid gameplay elements (like terrain deformation and good unit control) a great variety of units available to all players to master, and it handles nicely a dozen of players without population caps. The only problem is the small player base.