Baltar is far too obvious...and they've been playing along with he might be a cylon since the beginning..
Tigh was far too much of a stretch as it was, the fact Adama knew him from the first cylon war seems to have been completely glossed over...Tom Zarek would've seemed a good choice but considering he spent 20 years in prison prior to the start of the second Cylon war would discount him (although again that wasnt a problem with Tigh), or maybe the return of Ellen Tigh or Admiral Cain would be a good twist seeing how the 5th isnt part of the existing fleet
Tronsky