Protoss; the Covenant grunts run like little idiots. The elites would be covering behind a box then side strafe the zealots. By then it would be too late. Either way, zealots had their energy blades first. And the Covenant doesn't have templar...
The protoss would win because while the covenant posesses the larger ground forces, the protoss would win in any space battle. The protoss possess more powerful ships in greater numbers and historically have no problem with dying for a cause. Plus they think they're better than everyone else. The smaller covenant ships like the transports and banshees lack sheilding and hyperspace drives making easy targets for protoss vessels.
Protoss because I don't know who the hell commands a bunch of retards that are the covenant. I mean all the covenant does is bend over to get raped by Master Chief and his horde of fanboys.
Archons, Zealots, and Carriers could pound just about anything. But why compare the Protoss to the Covenent when the Zerg are the actual bad guys of StarCraft? And on that note, a zerg swarm would totally rape anything from Halo, biggrin!