Despite an indifferent start to the season when it seemed as if the Drivers' and Constructors' Championships would be contested between Ferrari and Red Bull, Verstappen showed his class to absolutely dominate the competition this past season.
At one point he was 46 behind Charles Leclerc after just three races before going on to win the Drivers' title by nearly 150 points in the end.
"I mean, 2021 was an epic year, and I think it'll go down in history - when we reflect on it in years to come - as one of the all-time great seasons of Formula 1," Horner said.
"I think that Max, having been through that and triumphed at the end of that… one forgets that he's still so young, you know,  years of age.
"Doing that last year, [and then] coming into this year, I think he just made another step.
"I think the maturity that he's driven with, he's just taken it to another level this year.
"You look at the Grands Prix that he won earlier in the year: the double-header in Imola was spectacular, his race in Miami with Charles… in those early races, the racing that was wheel-to-wheel was stunning.
"Then he just kept going, like a metronome, he kept delivering, and the more pressure there was, the more he delivered - [there were] some incredible victories.
"Hungary springs to mind, where he had to take a penalty and drive through the field; Spa was totally dominant; the pressure of Zandvoort… the list just goes on and on."