My favorite collaborator would have to be Kris Newton. I daresay that the sample adventure that he wrote for Adventures in Oz was better than anything that I could have come up with myself. While other issues have kept me from being as productive as I would like, at least part of why no further adventures have come out for AiO is that Kris is a very tough act to follow.
Our more recent collaboration, my appearance on the Gameable Saturday Morning Podcast discussing the animated Star Trek series (and Star Trek in general), was also a blast. It was also great to catch up with him, as we had gone from switching off GMing duties for a single playgroup to living in different states.