If we're talking in technical terms where the actual traits of the house are the determining factor, (e.g. Intelligence, cunning, loyalty, and bravery) Gryffindor is probably the most rare. Many people are intelligent, loyal, and cunning. I can't think of many in the general populous that are truly brave.

People of Harry Potter often want to join Gryffindor, and with good reason. This Hogwarts house values courage, bravery, and nerve. Plus, the book's heroes are associated with this house. Even Ravenclaw and Hufflepuff consistently choose to side with Gryffindor whenever it's feuding with Slytherin. (not specifically true)

Yes. One example of an evil Gryffindor is Peter Pettigrew, aka Wormtail, the servant of Lord Voldemort, who betrayed his “friends” Lily and James Potter to the Dark Lord by revealing the location of their house, which was protected by the Fidelius Charm. Peter Pettigrew was a Gryffindor and he was certainly evil.

But, he is only one of the kind of people in Gryffindor that were evil, but I can assure you, people in Gryffindor are what the house represents, which is good.