If you can afford it, UltraDns (Neustar) is the best. This is the service that Amazon uses, as well as a number of other very large sites. They have a 100% uptime SLA and a latency one as well. We haven't had an outage with them in almost two years. They also have an API which is nice. Oh, and they can do geo-distribution of requests, as well as a variety of other more advanced functions.
Last time I checked they charged per 1,000 DNS requests?