Here's what I ended up doing:
--Thanked them for the honor of admission
--Told them how much respect I have for their faculty/dept, and said how much I would have surely learned as a graduate student there
--Said that, after much deliberation, I chose X school
--Said that I hope we can stay in touch and I look forward to future contact with the fabulous students/faculty at their school
Sent this type of note (individually, not a mass email) to all of the faculty I met at each school I visited/was admitted to.