Gertie The Dinosaur (1914)
A lot of people seem to consider this the first "true" cartoon, animation did exist before this, but the line between 'first animations' and 'first animated film' is kinda arbitrary, and this is where a lot of people set that line.
This was animated before cels existed, so the entire background had to be re-drawn each frame. This cartoon was also the first to utilize 'cycling', repeating the same sequence of frames multiple times.
Little Nemo (1911)
Same director as Gertie The Dinosaur, you can really see how much improvement there was in 3 years. By the time Gertie was made spacing out frames to acheive easing was figured out.
Feline Follies (1919)
By this point you could start seeing the direction cartoons were headed in, that visual joke with the music notes turning into carts seems like the kinda thing Looney Tunes (which wouldn't exist for 11 more years) would do. Plus this one has an actual plot, with interacting characters and scenes that take place in different locations with new backgrounds. Though the animation of this time is still extremely rigid.
Humorous Phases of Funny Faces (1906)
At this point in time nobody's figured out spacing, everything moves very rigidly and always at the same speed. But even that was probably enough to blow peoples' minds.
Fantasmagorie (1908)
Same observations as above, but this one's actually funny, more importantly, it's funny in a way that actually utilizes the medium of animation. The hectic speed that these visual gags appear at couldn't be done as effectively in comic panels.
tries too hard to be edgy and is blocked by many because he acts rude towards others to feel better about himself.
