It's between the Danny Lopez fights or Wilfredo Gomez. No wrong answer here. Many consider the Gomez fight to be Sanchez's Magnum Opus performance though.
I agree that there are several to choose from. I might go with Looez 2, actually. Gomez was maybe more important in some ways, but Looez 2 was where he showed us he wasn’t a flash in the pan, he was officially the real deal, dominating again even though Lopez was slightly better the second time around.
Azumah Nelson. Yeah Nelson was a bit green but Nelson was actually a year older than him at 24. KOing Nelson late, after winning the fight and showing elite skills, is very impressive. Nelson went on to rule the division after Sanchez's death which shows us that Sanchez likely would have ruled that division if he decided to stay there. I know he had plans to move up. Comprehensively beating a man who went on to rule a division for 10 years is just a very very good win.