Charlie Chaplin - there seems to be some confusion as to which film is his debut. It looks as if it Making a Living which was made before his tramp character was fully developed

The first film made with his tramp character was his next film Mabel's Strange Predicament.

But his next film Kids Auto Races at Venice was released before Mabel's Strange Predicament. All 3 films were made in 1914

Buster Keaton's first film was The Butcher Boy (1917), where the lead character is played by Roscoe Arbuckle:

Cameron Diaz's real debut was of course her memorable entrance in The Mask (1994). (She did appear in something called She's No Angel made 2 years before but because of its nature  I doubt if a clip exists on Youtube)

A clip from Peer Gynt (1941) which is a very odd film but is notable for being Charlton Heston's film debut at the age of 17. He wouldn't make another film until 1950.

Plane Crazy is Mickey Mouse's debut:

Made before Steamboat Willie but not released until afterwards with an added soundtrack.

I happened to look at this thread and saw that all the video clips had vanished mainly because of Youtube having changed from http when I posted them to https.