Best Places to Co-Locate?

Stephen wrote in a while ago with this question: "Where's the best place to co-locate my Linux box in California? I heard there are loads of server farms in the San Fran to the L.A area. Which one is the best and most cost effective? Well most of us will agree that the Internet is much larger than California, so lets open this up a bit: What are the best places to co-locate ANYWHERE