Hazard scored twice in seven minutes before half-time, after Sol Bamba volleyed home for the visitors, to help Chelsea make it five wins from five so far this season.
He scored his third with a late penalty after Bamba brought down Willian, who added the Blues’ fourth from long range.
Speaking to the BBC’s Match of the Day, Sarri said: “Eden Hazard is unique.
“I thought he was one of the best in Europe. But now I am changing my mind as maybe now he is the best in Europe.”
Saturday’s victory means Chelsea have started a Premier League season with five successive wins for the fourth time, and first since 2010.
Defeat for Cardiff extends their winless run to six games, their worst spell since Neil Warnock was appointed manager in October 2016.
They started well at Stamford Bridge, and deserved the lead given to them by Bamba, but Chelsea’s quality ultimately shone through.
The Blues have five points more than at this stage last season, but Sarri still wants his side to improve as they prepare for a busy two weeks during which they face second-placed Liverpool twice – in the league and EFL Cup.
He said: “I am very happy with my players. But I always think we can do better. We are very dangerous in the offensive phase but we aren’t as solid defensively as I want. I think we can improve quickly, though.
“I think at the moment Liverpool and Manchester City are another step up, so we have to improve, and I hope next season we can be on the same level.”
Chelsea came into the game in much the better form, but it was Cardiff who dominated the early stages – much to the hosts’ frustration.
But, after Bamba’s opener, Hazard changed the game with a low 37th-minute strike and doubled his tally with a heavily deflected shot after Olivier Giroud did well to lay the ball to his team-mate.
The Belgian wrapped up his hat-trick late on to seal victory for a Chelsea side who rarely looked in danger of conceding in the second half.
After goals against Newcastle and Bournemouth, as well as against Scotland during the international break, Hazard has scored six goals in his past four games.
He had appeared to be on the verge of leaving Chelsea in the summer after six years with the club, but new boss Maurizio Sarri is getting the best out of him.
And his performance meant Chelsea did not need to rely on a late goal – as they had against Arsenal, Newcastle and Bournemouth.
