AT&T has the biggest total number of subscribers. Verizon Wireless has the most direct subscribers and seems to be signing new users faster than AT&T (although iPhone may've thrown that outpacing off a little bit).
Wirelessly posted (Walkguru's: Opera/8.01 (J2ME/MIDP; Opera Mini/3.1.8295/1716; en; U; ssr)) i think att. but im biased.