Ronald D. Moore has already said that the fifth Cylon does not appear in the "Last Supper" photo, he even went further to say that the seat next to Lee is empty. You don't allow image tags so you're going to have to google if you want to see it, but in the photo, from left to right, we have Laura Roslin, Caprica Six, Saul Tigh, The Fifth (empty), Lee Adama, Gaius Baltar, Hallucination Six, Kara Thrace, Sam Anders, Galen Tyrol, Athena/Boomer, Helo, and Bill Adama. Assuming that leaves us with only main characters (and let's assume living ones) who haven't bred with Cylons we're reduced to:
Felix Gaeta
Anastasia Dualla
Tom Zarek
Romo Lampkin

It should also be noted that the empty seat, in the Da Vinci "Last Supper" was occupied by none other than Judas Iscariot.

That leads me to believe that it's probably Felix Gaeta, since he sold out Adama and Roslin during the election. It also gives us a fair reason to eliminate Dualla, and I think Zarek can be eliminated simply based on who is playing him. Lampkin is a relatively new character, and I don't really think he's much of a Judas, but I wouldn't rule him out. My money is on Gaeta, but Lampkin wouldn't surprise me.