I had a very successful time as Roslin not revealing myself. We had a sympathizer that was up in the ressurrection ship causing bad times... so bad in fact that the other players were wondering what the other cylon was waiting for. Well, if they had just pondered their question a bit they would've easily concluded that I was the cylon... basically getting the actions of caprica AND the activate they cylon fleet locations, plus the added bonuses being able to throw as many skill cards at checks as I wanted(but I had to be careful here) and keeping paranoia high so the the two humans were reluctant to play as a team since they weren't completely sure who was who. Who are your favorite people to be as a Cylon?

Very clearly being president or admiral is a prime Cylon occupation. Being able to steer the fleet into hazardous locations while witholding nuclear action, or being able jail your enemies and spread rumours about the other players' loyalty cards is the way to go.

In terms of best actual character, Saul Tigh is pretty savage. He has an easier time jailing people (considering he has the bonus on his character sheet and also naturally draws Declare Emergency), can seize the presidency as an action once per game, and is second in line for admiralty (Saul can jail his old pal Bill (if the old man is even in the game) with his special ability and use Declare Emergency to essentially claim the Admiralty).

Of course, you'd never realistically be able to actively claim both titles in one game, other players might take issue with having all the eggs in one basket. Still, if you're a Cylon as Saul, and someone uses Executive Order at a critical time, Saul can turn things against the humans very quickly.