It depends whether or not you are talking about to the head or body. Hatton and Cotto have excellent ones to the body, whilst DLH and Henry Cooper are two of the very best head punchers.
Monzon's left hook was beautiful, a similiar styled moderner in Kessler has a similiar crisp left hook that looks great on film and catches the opponent coming in well.
Cuevas' Left hook broke bones! Ruben Olivares had a devistating left hook also. As well as Joe Frazier. Alexis Arguello, the explosive thin man.
Bert Cooper and everyone is forgetting Tommy Morrison. There was a fighter named Angel Vasquez back in the day that reminded me of Tito's style of throwing hooks. I wonder if any of you guys remember him. I believe he fought at featherweight.
Oscar gets no love on this subject. His left hook is a thing of beauty. Maybe not the best, but should definately be up there with/instead of a few of the fighters in the poll.
Out of those listed, only Frazier had a REALLY standout left hook. The others had excellent hooks, but they weren't death sentences. For example Tommy Morrison's hook I would rate better than Hatton's. And where is Trinidad? The man nearly built a career on his hook! (overstatement but you get my drift).