What is the largest moving company in the US?

The largest us moving company is United Van Lines, both in size and market share.

Also wanted to add; Just because a company is the biggest, doesn't necessarily mean it's the best. I would check other options as well. You might want to try searching for companies and/or websites that provide you with a bunch of listings so you can shop around in one spot for a moving company that meets your needs. I find that a lot of the family-owned moving companies have better customer service and are more careful with your stuff, but maybe that's just me.