Some people say Deathstroke, but he has been an ally to them in the past, and there was a time in the 90's when he was not really a bad guy, so I'm not sure if that counts. Most of the stories with him that I read have him as just being a mercenary, so he mostly does the jobs for the money. He seems to go back and forth though, in terms of being a villain or a semi-villain. I guess that he's written differently in his own series. I started reading the Titans when Geoff Johns wrote and relaunched the book, and Deathstroke was definitely a villain for the team then. So, is he their number one? I know some other enemies of the team are Brother Blood and Trigon, they seem to rank pretty high. So,does the team have a Joker or a Green Goblin?