I am halfway through the Boethiah's Calling quest and have just wiped out all of her followers which seems the right thing to do. I'm now wondering who is the most evil follower to sacrifice? I was thinking about hanging on until after I've finished the DB questline and then using Cicero however I have read that he can't be sacrificed and have also read that he can. Can anyone tell me once and for all if this guy can be sacrificed or not?

If not I can only think of Jenassa in Whiterun who seems to be pretty evil based on her dialogue. Can anyone give me any other suggestions? I don't care about who is the most powerful/useful follower, just who is the most evil for my good guy character to sacrifice without guilt.

No, you cannot sacrifice Cicero during the quest. Doing so will cause the quest to become bugged, because he is an essential follower.

If you want to sacrifice someone who is "evil", then http://www.uesp.net/wiki/Skyrim:Eola would be a good choice.