Till the current date, the best rated broom of all by the Magical Counsel of Broom Standard & Specifications is Firebolt Supreme although there are some close competitors like Thunderbolt VII, Firebolt, Transylvanian Burb, Varapidos etc.

And it's rating is as given in its specific chapter :

Speed:10/10

Control:9/10

Durability:10/10

There is a rumour (some old wizards say actual lost fact, some young ones doubt it as a myth though) that there is one broom whose Riding wood was made of Elder wood and wrapped in snake skin. Legend says it has some long forgot unbreaking charm all over it's body, along with super aerodynamic structure at its end using Norwegian ridge back's ridges to counter all the air friction. Paddle was made of basilisk's fang. Broom tail was made of unicorn hair wrapped tight by stitches using chimera claws.
According to rumour it is the most powerful broom having maximum strength, agility, friction & vibration control, speed & acceleration having a rating of

Speed:10/10

Control:10/10

Durability:10/10

There is a story that goes along with the rumour of this broom.
Some sayings are there that depicts this broom as the one carried by death itself to reach from place to place in no time. Although another story says that death is capable of apparating anywhere anytime.

Some believes that death had this broom when he countered the three wizards of "Tales of Biddle the bird" across the river bridge.

There is no one living in this modern world to validate all these rumours though.