Frankly, I'm still with the 13th Age Core Book. And it had to also have all the DMG and MM stuff in it.
It had the Rules Glossary before 2014 5e was originally released It was fantastic. That's now 11 year old idea, having it at this point is just keeping up with the Joneses.
It had different information about the classes, like a complexity guide for players.
But most of all it had sidebars from the two designers explaining why the rules were what they were, and consequences of changing them. They even had cases why where they differed and why they ended up with this rule. It made hacking and adjusting the game so much easier, and without that designer dive into the why of the rules, it's very hard for any other rulebook to compete.
Oh yeah, and art, layout, etc.
Not putting down the 2024 PHB, but it's missing some things to be considered "a masterpiece", and other things were praiseworthy when they came out before 5e, but are just "not falling behind" 11 years later.
EDIT: 13th Age was a d20 OGL, made by a lead designer of 3ed, Jonathan Tweet, and the lead designer of 4e, Rob Heinsoo, as the game they would want to play in their weekly game, and as a "love letter to D&D". It also adopted a streamlined approach, much like 5e followed with, though was able to break with certain sacred cows since it wasn't "D&D" as a brand.