I usually write to whoever interviewed me, and then whoever gave the majority of the information talks (usually it's the Dean, but it could be a member of the admissions committee or some other administrator), and anyone else who I had a lot of contact with for that day. I usually send 3-4 thank yous from each interview, but I like writing letters...it's definitely not a suckup thing. (And I removed any dignity I MIGHT have gained by only having Lion King stamps left!! 😳 )
I'm REALLY bad with names, so at the end of every interview, I sneak into the admissions office and hope that no one sees me while I ask the secretary for all the names and correct spelling of the people I want to write to 🙂 THey usually understand, and there's usually a few people doing the same thing.
If students gave tours, and they gave me their cards or someone else passed along their e-mail addresses, I usually mailed them an e-mail thank you (the other people got thank you cards). Most of those wrote back encouraging me to ask any additional questions - they love helping out us terrified applicants, whom I'm sure they can heavily relate to.