An obscure choice but Yuki Nonaka is a very nice pure boxer. Sadly his lack of other traits means he'll never make a mark outside of the Japanese scene.
For starters... Harold Johnson Willie Pep Nicolino Locche Pernell Whitaker Miguel Canto Sumbu Kalambay I do agree that FMJ and Rigondeaux are the best pure boxers right now. I'd probably add Ward as well.