I'm a die hard fan of the team and given the modern/Free agent era we are in, it's remarkable and not likely to be repeated - Sorry Patrick Mahommes,
But, even I would have to say the Celtics winning 11 of 13 seasons from 1957-1969 would have the be the gold standard of excellence. For a close second (and maybe even first given how hard it is to stay at that level of conditioning, I might have to go with Michael Phelps Olympic run. One thing to compete in an event or two, but in as many individual ones he did and win gold - jeez)