My personal favorite is Nacho Beristain. I think a lot of the greats have been covered here - Futch, Arcel, Blackburn, Benton, Steward and I seen Brusa mentioned by surfinghb. He’s got to be up there. I’d also like to mention Arturo “Cuyo” Hernandez. Molded many great Mexican technical boxers like Finito, also had Olivares and Zarate and was something of a mentor to Nacho. He died a few months after Finito won the title in 1990 hence how Nacho took over the reigns as his trainer.
Hugely important earning he respect of your fighters. From my readings Blackburn certainly had that. He's an interesting story that's for sure.
I think by far, the most under rated guy is Sdunek. Look at that stable of boxers he had and all those title bouts.