Joe Frazier and Miguel Cotto are the first that spring to mind. Odd, given both fought from an orthodox stance. Of Southpaws, Corrie Sanders and Adonis Stevenson could be somewhat left hand happy.
Broached this topic a bit more broadly several years ago in my One Armed Bandits thread if anyone wants to check it out: https://www.boxingforum24.com/threads/one-armed-bandits.585909/
One of my favourites, John Conteh, already mentioned but close runner-ups: Tony Sibson Henry Cooper Cyclone Hart Tony Baltazar
Of course, my favourite fighter of all-time, Joe Frazier, will feature prominently here. There are some colossal myths in Boxing…none more persistent than the lie that Frazier was one-handed.
For reasonable periods of many of his fights, Holmes would exclusively use the left jab to soften his opponent up before truly mixing his shots. He jabbed Kenny Norton to death for about the first 3rd of their fight IIRC. So much for Larry’s impaired left arm, an alleged handicap during the Norton fight that is inexplicably claimed every now and then. We should all have left arms that are so impaired. For the first 5-6 rounds or so, you could photoshop the rest of Larry’s body out of the vision - the left hand was doing most, if not all, the work. Imagine seeing this left arm, all on its own, eerily suspended in mid air, constantly pumping punches into Ken Norton’s face. That’s a Twilight Zone story/episode begging to be made - The Golden Arm…that’s if any would-be film maker is willing to go out on a limb - get it? As always, I just trade marked the idea.
Johnny Famechon boxed almost exclusively behind a peach of a left jab (and a decent left hook) with his right glove almost permanently attached alongside his chin.
I bet a lot of people don't know Joey Maxim's surname because of his left hand. Berardinelli. just so when folks didn't know who Giuseppe Antonio Berardinelli "Light Heavyweight Champion of the World" was they crowned his grave with "Joey Maxim"