Sounds like you guys are East Coast and West Coast and Cingular some how missed the South and Central States.
But good coverage seems to have a lot to do with all of the mergers and aquisitions. Take Verizon, for example. They bought GTE, U.S. West, PrimeCo, Bell Atlantic, etc. So in certain regions, Verizon is going to be better than others.
Same is true with AT&T. SBC bought AT&T, then AT&T bought Bell South. So if you were in an area where SBC or AT&T were good, then good for you. Down here, Bell South always sucked. The only reason people used Bell South was so they could get cell service on their home phone bill. Now that AT&T is Bell South, that suckiness transcends to AT&T.
As for Nextel; there's a reason why Sprint keeps them around. One, their network is not compatible with Sprint's (IDEN vs. PCS) but also there is such a large network already in place as well as the largest, fastest and most reliable PTT network that they'd be crazy to change it or shut it down.
I think in all my years, Verizon has had the best over all coverage, so I would assume that they have the most antennas. But that opinion doesn't apply to everyone since they don't have towers everywhere.
I think a good link that shows how silly the discussion of "what cell phone company is the best" on a global website is here:
http://www.cellreception.com/
For example, if you look up my zip code, there's a bunch of people in one of the three cities that use that zip praising Metro PCS. One person in a second city said they switched from Metro PCS and switched to Nextel and now they're very happy. When you go to the map of towers, you find that the guy praising Nextel over PCS is about 100 feet from a Nextel tower. And as I drive away from that tower, I can tell you of two precise points where twice I completely lose signal.
As for who actually "owns" the most towers; that's a loaded question. All of the companies own their own towers, but not all of the towers are owned by cell phone companies. There are several companies that erect towers and then lease out spots on the tower. These towers will have several company's antennas on them. Use the antenna locator at that site. Lots of good fun.