Pac's career arc has to win this. Nobody has done that before or since. It may never be repeated again. To go from an undernourished youth who stuck rocks in his pockets to make the MINIMUM weight for mini-flyweight to a man who moves all the way up to welterweight, winning 8 titles along the way, wow, that's something.
All impressive. But winning the heavyweight title at 20, after less than two years as a pro, given his size (as well), is pretty off the charts ... Particularly since the older Mike Tyson gets, the smaller he looks ... compared to how big the heavyweights are today, and how old heavyweights seem to be today before they do anything. I'm like a year or so older than Tyson, and I remember being in college when he beat Berbick. And we were all just stunned someone our age (or younger) had accomplished that. And it wasn't just that he won a belt, he won all of them and beat all the reigning and former champs around him. A thorough housecleaning. I'd put Joe Louis and his long reign a close #2. (On another day, it might be #1. They're close.) Five of Joe's 12-year-reign were mainly exhibitions. And Wlad came close to matching it recently with his nearly 10-year reign (with no years off). Joe's reign I can see being surpassed one day. Same with older guys winning titles and someone winning more belts in more divisions (there's always a division being added somewhere). But I don't know if we'll ever get a 19-year-old heavyweight champ. That seems a stretch. You're not even fully grown yet. He'd have to be some phenom.
The order of achievement—Featherweight, Welterweight, Lightweight— is impressive too, especially in a time when day-before weigh-ins didn't exist.
I'll give this to the only 17-year-old world champion we'll ever see, Wilfred Benitez. Props to Antonio Cervantes for agreeing to be part of a "no-win" situation. He literally fought a minor. Meanwhile, the current fight scene has guys like Ennis, Ortiz, and others who established champions consider too young/inexperienced to lock horns with.
Louis. Pac is about even but he kind of bullshat his way to 135 and 154 titles. BHop hung around by beating B and C listers. Tyson’s unification at a young age goes a lot further than his youngest heavyweight champ claim.
Winning the WW title from an ATG whilst weighing under the LW limit always impressed me. He was under the LW limit for a few of his WW defenses too, though none against a fighter as brilliant as Ross.