Gertie the Dinosaur

Gertie the Dinosaur is an animated short that was created by Windsor McCay back in 1914. It is considered to be the first ever animated movie to use the techniques of keyframes and tracing paper. When it was first shown, it was an interactive experience where McCay himself would interact with Gertie and Gertie would do tricks. Unfortunately, McCay's employer at the newspaper he produced comics strips for realised his employee was beginning to turn his attention away from comic strips and onto animation so he began to steer him away from the field. That is why McCay recorded himself interacting with Gertie and then this live-action footage was shown alongside the animation.

To me it is easy to understand why the short is so highly regarded, just from a technical stand point the animation holds up to this day and considering it is the first short to use keyframes and tracing paper the timing is fairly spot on.