I can see where people are coming from who answered “feet” and “horses.” I guess it depends on whether you’re asking from a historical aspect or effectiveness. Horses and feet have certainly been around the longest, but they’re rather limited in comparison to motorized vehicles. You can’t drop bombs and wage war across oceans on horses.
But this is all very relative. Back in the day, being on a horse was a huge advantage over a foot soldier. But now with the evolution of military technology you need to get more and more out of your transportation. Tanks were revolutionary in warfare because they were highly protective and destructive at the same time – much more efficient than any other technology available at the time. I’d also say some of the planes developed during WWII rank way up there. If not for our advancements in airborne warfare, we may not have won the war in the Pacific.
I’m by no means a military expert, by the way. My dad just watches a lot of military shows on the History channel, and I absorb some of that info by osmosis when I’m at home.