It's close for me, with Slytherin in the lead and Gryffindor not far behind.
Slytherin
Head of House: Posie Parker.
Slytherins tend to be ambitious, shrewd, cunning, strong leaders, and achievement-oriented.
Other alumni include: Graham Linehan, Germaine Greer, Julie Bindel.
House Ghost: Margaret Thatcher.
Gryffindor
Head of House: Magdalen Berns.
The Gryffindor house emphasises the traits of courage as well as "daring, nerve, and chivalry," and thus its members are generally regarded as brave, though sometimes to the point of recklessness.
Other alumni: Venice Allan, Sharron Davies, Meghan Murphy.
House Ghost: Emmeline Pankhurst.