I absolutely could not STAND Cicero. I understand that he was really loyal to the Dark Brotherhood and everything, but do you really need to attack everyone and try to kill them? And he was just so annoying. Always talking in third person with that unbearable voice. Guhhhh I'm glad I killed him.
That being said, I think Astrid pissed me off more than Cicero did. As soon as I ran out of the Palace after poisoning the "high king" and saw the guards surrounding me, I knew it was Astrid who had betrayed me. Whatta bitch.