If coverage/connectivity is your main priority, I'd say VZW is number 1, followed by AT&T.

TMO and Sprint are the affordable options, particularly if you have multiple lines and are in larger metropolitan areas, but really suffer in those spaces in between, in my experience.

EDIT: I do think VZW has better coverage than AT&T, but my company has preferred relationships with both. None of my coworkers have had any issues on either carrier in most places (aside from inside some buildings, where TMO and Sprint are practically useless).