Really? I only remember Mayweather say he'd retire once and he did.
I'd have to say Penn too.
He announced retirements after Baldomir and again after Hatton
http://www.espn.com/sports/boxing/news/story?id=3429729
If you search for interviews, he never officially retires but almost always states he's set in life and considers retiring every day. Some interviews, he gets fed up with constant, nagging questions(about Pacquiao) and straight out says he's retiring.
Of course, he retired after Berto but just earlier this year said he'd come back for a fight against McGregor.