Most of the teachers? Binns and Trelawney certainly, Snape, but arguably he's more unprofessional and unpleasant than incompetent. Often the defense professor is incompetent, I suppose. Canon!Harry had Lupin, Snape and fake Moody for competent defense professors and Quirrel, Lockhart and Umbridge for incompetent ones. We have no reason to doubt the teaching ability of Mcgonagall, Flitwick, Sprout, Sinistra, Vector or Babbling. Burbage's Muggle studies course is often a subject of ridicule in fanfiction, but that might be a result of the (inter?)-national curriculum rather than her individual competence, and so would be no better at other schools. Hagrid's Care of Magical Creature's lessons were of very uneven quality, but he could teach well when he had his head together.

As to the preeminence of Hogwarts, perhaps its as simple as Hogwarts being the only British school with a comprehensive curriculum, the others focusing on particular areas of magic and functioning more or less as magical trade schools. We don't technically know that there's no entrance exam for the common witch or wizard, we just know Harry didn't have to take one, he could have been admitted as a legacy student or simply because he's the boy-who-lived. Or the barrier could be financial.