Any enterprise operators very happy with their MPLS providers?

I'm getting ready to prepare an RFP for our next generation WAN, and would like feedback from anyone else who has 100+ MPLS nodes on their quality of account service and technical performance. My current landscape includes AT&T, Sprint, and Verizon. I'm almost completely happy with Sprint- they're about in the A- range. AT&T is muddling along at about a C, and Verizon is a solid F. I've heard very good things from some CenturyLink customers and will definitely include them in the bidder list- is anyone else doing a very good job for you? -Gabriel