Verstappen finished the Las Vegas Grand Prix in fifth place, one spot up from title rival Lando Norris.
It was a result that confirmed the Dutchman as champion courtesy of a 63-point advantage with a maximum of 60 points on offer with two races remaining.
“This one is a very special victory because obviously there has been challenges this year,” said the Red Bull Racing team boss.
“But I think that Max, he’s scored more than double the amount of race wins than any other driver. He’s won it with two races to go.
“He’s been head and shoulders the best driver on the grid in Formula 1 and I think what he’s really done this year is absolutely cement his position amongst the greats in the sport.
“Both statistically and on the way that he’s driven, that’s unquestionable.”
Verstappen won seven of the opening 10 grands prix but, since the Spanish Grand Prix in June, has found the going much tougher.
He’s had just one race win since then, the Sao Paulo Grand Prix, and the Sprint victory in the United States.
According to Horner, that prompted the Dutchman to work harder than he had in any of his three previous titles.
“The last couple of years, he’s won at a canter; this one he’s had to really dig deep,” Horner reasoned.
“He’s applied himself. He’s worked hard. He’s put the mileage in. He’s gone the extra yard with the engineers, spending time talking to the designers, to the R&D.
“There’s 1000 people in our team that make these incredible cars and you need all of the departments to be working collectively to deliver big results.
“What Max has done is he’s really helped to inspire that and given phenomenal feedback and direction from what he’s needed when the wind tunnel wasn’t correlating with the track.
“It was his feedback that then enabled us to focus on ‘this is the area you need to work on’.”
Verstappen suggested his title win came despite not having the fastest car for more than half the season.
It’s a statement Horner agrees with, suggesting McLaren had the upper hand since Miami.
“That was a turning point,” Horner conceded.
“The car was difficult to drive, it was very difficult to drive, but he was able to adapt and get the most, get the most out of it.
“I think we’ve improved it in latter races, but certainly around Monza time, it looked like the championship was slipping away.
“We started the season very strongly, but by Miami, it was clear that McLaren had made a significant step,” he added.
“But Max, when you look back at the season as a whole and you think back to the races he won in Imola and Barcelona and Montreal, that were all very tough closely fought races.
“Through the summer months, when we were struggling a bit with the car, he was still second in Silverstone, second in Zandvoort, he was still picking up big, big podiums and results.”
Those performances and the growth of the Dutchman during the difficult mid-season stretch suggests to Horner that there’s still more to come.
“He’s only just 27 years of age,” he noted.
“I think there’s still, with how hard he pushes himself, I’m sure that there’s still more to come.
“He’s got the capability to do it,” Horner added when asked if Verstappen could surpass Lewis Hamilton and Michael Schumacher’s record of seven titles.
“That’s dependent on other factors.
“This sport is about man and machine, so we’ve got to make sure that we’re giving him the machinery that’s capable of competing.”
Verstappen has joined an elite group that includes Juan-Manuel Fangio, Alain Prost, Schumacher, Sebastian Vettel, and Hamilton as the only drivers to have won four or more championships.
