"Now the man Moses was very meek, more than all people who were on the face of the earth." (Numbers 12:3)
It does seem oxymoronic for Moses to refer to himself as the humblest human being in the world. But there is more here than meets the eye.
Firstly, Moses does seem to be possessed of an inherently underdog nature. Reared in the royal courts of Pharaoh, Moses could've lived life in the lap of luxury. He had the American dream down. But when he saw a fellow Israelite being bullied by an Egyptian, he didn't think pragmatically, counting his set life as a deterrent to doing the noble thing. He shed Egyptian blood and left easy street for a life of obscurity in the desert, becoming a good son-in-law in the tending of sheep. Imagine the lowliness of mind and self-estimation required for such a transition!
Also, Moses appears to have been afflicted with stuttering. As a stutterer myself, I know firsthand how humbling that can be!
Secondly—and this is perhaps the weightier point in the understanding of the passage—"meek" here can mean "miserable" or "burdened." God's call upon him signaled the beginning of a life of carrying the burden of the people of God. So when Moses refers to himself as the meekest man on the planet, what he's really saying is that his role in redemptive history is such that the weight of care and trouble that this mandate brings far exceeds that of anyone else's "stresses." Considering the fact that Moses is a type or shadow of Christ, it does make perfect sense.
A fuller discussion here and here.