I'd have to say, infantry. I was a hardcore Zerg player in StarCraft, and my most used (and often abused) unit was the Zergling. With all melee, carapace, and zergling specific upgrades, a horde of 48 Zerglings could strip even the mightiest Protoss bases to their foundation within mere minutes. Not even the Terrans' vaunted Firebats coul stop my incessant rushes of overwhelming numbers.
Infantry make excellent fodder; why focus on skill - something hard-earned and 'expensive' - when you can send wave after wave of bloodthirsty, undiscplined warriors to bash through enemy ramparts through their sheer numbers. Yes, it's dirty and icky and bloody and gooey, and you need lots and lots of people, but that's accomplished easily enough, and nothing spurs the masses on quite like seeing their enemies drown in pools of their comrades' blood? :devil
That, and I also see the effectiveness of the 'single body of soldiers' tactic, employed quite successfully by the Romans, for example. They simply bore down on the enemy and used their collective mass to suffocate the enemy forces.
Quite effective, either way. :yes