In the books, Rupert Falwell, Loras Tyrell, or possibly Ned Stark.
Rupert Falwell, or the Fighting Fool, was a historical character who killed two Kingsguard at the Battle of the Great Fork during the Faith Militant uprising.
If we're counting "unofficial" Kingsguard such as Renly's Rainbow Guard, then Loras Tyrell makes the list for killing two of his own Kingsguard after they failed to prevent Renly's death.
In the fight at the Tower of Joy, it's never made explicit exactly who killed who. Since Ned and Howland were the only survivors, it's reasonable to assume they killed at least one or two of the Kingsguard.
As far as I know, nobody in the books is known for sure to have killed more than two Kingsguard.
In the show, Sandor Clegane.
As you note, the Hound killed four of Cersei Lannister's Queensguard plus one former Kingsguard all by himself, making him easily the victor of the Kingsguard Killer kompetition in Game of Thrones.