Haha, yeahh...
I think they are the biggest on a large scale now that they acquired Cingular. Cingular had a huge network already. Verizon is only really good on the east coast U.S. as far as I know. I remember on trips to New Mexico and California, service was crap. The reason they dominate the east coast is because of their IN plan. I got unlimited IN calling and texting, and I've never gone over my plan since.
One other thing about the iPhone - it requires at least a $60/monthly service plan to use.