Classic maiden

Oaks - Sun Princess - 1983

Derby - Merry Hampton - 1887

Don't know about the Guineas ... and then there may  well be Irish, and even French races, qualifiers - especially of the equivalent to our Guineas.

Lady Capulet won Irish 1000 GNS on  her debut in late 70s

Snurge was a maiden when he won the Leger in 1990, though he shouldn't have been. He'd won a race in France as a 2yo by 3 lengths but got DQ'd. He then got beat a head by Belmez in the Voltigeur, having been beaten a head on the Saturday of Ascot's June meeting in his only previous race as a 3yo. Not sure if there's been another British maiden winner since. There were many years ago, when horses were often kept under wraps and trialled much more at home.