Alright, I wanted to expad a bit on what I said.
There are really five people I do not want to see be Cylons, and I will be upset.
1) Billy, since he was by far the heart of humanity. His being a Cylon would make Roslin's journey into darkness meaningless, since it was his death that started it.
2) Speaking of, Roslin is the second. She's the mother of humanity, and Cylons don't have children. It completely destroys the idea of Roslin/Adama as mother/father figures.
3) Gaeta. Mostly sentimental, but he also fills a "human" role - he's by no means perfect, but he does the best he can.
4) Ellen. Her being a Cylon renders Tigh's journey just as meaningless as Billy being one makes Roslins.
5) Baltar. Like Billy (and Gaeta, to a lesser extent) was everything good about humanity, Baltar is pretty much everything bad. That dichotomy is central to the show - humanity as a whole isn't good or bad, but people can be. He is the anti-Billy.
Now, who would I like to see be a Cylon?
1/2) Alosha, and/or the Oracle. I like the idea of the Cylons subverting faith to guide things.
3) Tory. Billy is killed, and Roslin turns down a dark path. Tory as a Cylon is a great representation of that.