I, at one point or another, have had ALL of them.
AT&T was probably the worst I have had but that was 10 years ago. I have had Verizon for 9 years and did the "Try it for 30 days" from Sprint, Nextel (before it was Sprint) and T Mobile. All of them sucked.
I hate Verizon's customer service but find that they provide the best service throughout the US. I travel for work about every other week and my Verizon service has been great, everywhere from Iowa, Alabama, Arkansas, New Jersey, New York and all up and down the California coast.
I will say to stay away from the HTC phones. I switched from a BlackBerry to the HTC Ozone (Windows Mobile) phone about 18 months ago and it has been the worst phone I have ever had. My Droid 2 will be delivered today and I am VERY anxious!