I've had direct experience with them all except Sprint, which I have worked wit in a 2nd hand nature.
This is rating them purely for the quality of the transit and reliability, etc.
Verizon/MCI, Sprint, Level3, AT&T, and Qwest.
Level(3) used to be number one, but their stretch of de-peerings in 2005 followed by increased network instability has forced me to drop them down the list a little bit.
AT&T just plays the game of not increasing peering to various carriers to try to force them, or others, into buying AT&T transit. Really, the only way to assure good routes to AT&T end-users is to connect to AT&T's network. Use anyone else and you risk congestion issues we've even seen AT&T play that game with Level(3). On the other side, if you have an AT&T line, expect congestion at various peering points because of this policy.
Qwest just isn't in the same league as the others. Last I checked, which was back in 2003, they may have had the best support of the bunch, but the quantity of peering locations and the nationwide backbone didn't compare to the others listed. Still, a very good network, still a tier 1, etc.