Coverage. Coverage. Coverage.
If I had to pick one for coverage in CT = AT&T because it seems to slightly edge out Verizon with signal reach (and I'm thinking that might be unique to CT).
Customer service seems best with Verizon.
I have a T-Mobile account. When it is within signal range it is absolutely the best voice quality of all my phones. I'm using a Google Nexus S on T-Mobile
Be sure you review:
http://business.t-mobile.com/
Hard to beat $44.99 for unlimited
The current ""best"" Android is the Samsung Galaxy SII
I also have the BlackBerry 9930 which I like quite a bit primarily for the keyboard and secondly for the fact that I paired my Gmail account with a $10/mo BlackBerry Exchange to REALLY sync email and it is much better than stock BlackBerry Gmail sync .