The Celtics. Yeah, they're good but you have to take into consideration that...
Miami, Cleveland, and Indiana were all short of their best players. I believe that a fully healthy Miami team could've gone six games. The same thing with Cleveland if Mitchell didn't miss games 4 & 5 on top of Allen missing the whole series. Indiana had an opportunity to possibly win 2 or 3 of the first 3 games but inexperience and questionable coaching/defensive decisions fuked them in the end.
Dallas was the only healthy team they've faced but unfortunately, Kyrie hasn't played well and the supporting cast/bench's luck ran out and they've laid a goose egg in the Finals.
At least Denver faced teams who had their best players available. One of the reasons why Tatum has had a lackluster postseason this year is because he's taking it easy knowing that the competition is short-handed and his team being great as they are he can afford to play subpar and they'd still win.