Zerg have the hive mind. Plus, all the variants of the Zerg are species that have been converted into whatever the swarm needs. They would just have to begin assimilating (BOOOOOORG) the various controller species and it would be game over. The Zerg are parasitic and all that, but it's more hardcore than the Yeerks. As far as space travel, the Zerg are able to tear open rifts into "warp space" and move quickly to other planets, and their method of travel is so precise they can pop right into the atmosphere without leaving any trace of their entry, if I'm not mistaken. The quickness of their travel would definitely be a huge advantage.
Though I've always loved the Zerg, but it's mostly because I'm a big fan of insectoid races (Zerg, Tyranids, Buggers, Akrid, the Brood, the Bugs, etc.). They're just so awesome...