This is a very good question and a good test as well. IT's hard because there are so many great Mexican American champions. LOL, I feel almost inclined to Cheat and add my favorite Mexican fighter, Julio Cesar Chavez, but you said "Mexican-American". He would top this and many of the lists I've seen here but, sticking to the question, I'd have to go with..........
1. Oscar de la Hoya: Say what you will but it's hard not to admire Oscar. I must admit that I personally like him more for what he's doing for boxing after his retirement. I also have to give Oscar a LOT of credit. He didn't always WIN the big ones but he did FIGHT them. Some will say he avoided Winky Wright and Vernon Forrest but at nearly every time they were available, there was always a more attractive challenge waiting for Oscar. He's the one fighter who can ligitimately claim to not be able to fight Everybody.
2. Diego Corrales: How can you not admire Chico? He didn't always fight the wisest fight but he certainly fought the bravest. I felt for him when he wanted to continue when his step-dad stopped the Mayweather fight. I felt for his dad as well though because he understood that Chico was out-matched. He should have been a jr. welterweight, not a jr. lightweight.
Orlando Canizalez: a good champion with a lot of heart, so is Michael Carbajal. Who can forget his series with Humberto Gonzalez?
Honestly, I've never been impressed with Fernando Vargas or Jesse James Leija. They were "Good" but ordinary champions who benefited from the Multi-titled generation and would never have become champions of any sort in, even the days of TWO champions per division.
Chacon, Baltazar and Paulie Ayala were decent champions but, again, nothing special.
People tend to remember Carlos Palomino for nastalgic reasons but he benefited from the company he kept during the welterweight division's super era with Leonard, Duran and Hearns. He's the "Ken Norton" of that era.
There are other good champions on this list and the lists of others here. But they pale in comparsion to their Mexican counterparts.
LOL, I feel cheated, not being able to add names like Chavez, Sanchez, Marquez and others but rules are rules.