The Miz became the first wrestler in WWE history to become a two-time Grand Slam Champion at Elimination Chamber 2021.
As pointed out by Twitter's @WWEStats after the show, beating Drew McIntyre to become WWE Champion for the second time makes Miz the first wrestler to ever complete the full round of belts needed to be classified as a modern Grand Slam Champion twice:-
The A-Lister's second World Title reign began almost a decade after his first ended, as he cashed in his Money In The Bank briefcase to pin McIntyre, who had just competed in a gruelling Chamber match. Bobby Lashley destroyed Drew after that bout, softening up for Miz to pick the bones.
Other wrestlers classed as Grand Slam Champions by WWE's modern format include Kurt Angle, Edge, Chris Jericho, Roman Reigns, and Daniel Bryan. In terms of raw accomplishments, none can say they have done what Miz has done.
You'd never be able to tell it from the way Miz has been booked over the past year or so, though:-
Miz cut a fired-up promo after Elimination Chamber, blasting "internet fans" who claimed he didn't deserve to be WWE Champion.