My largest pixel art project so far has been a mod for the game Enter the Gungeon by Dodge Roll. I put in well over a hundred hours into it, as one character requires a LOT of work.
While the project wasn't completed in its entirety, it's definitely at least 75% there, and where I stopped was due to technical limitations and coding problems I don't have the understanding to solve rather than actual motivation at the time. I would love to get to work on another large scale project like this.
A smaller project I attempted was the sprites for a character mod for Risk of Rain: Returns by Hopoo Games. Studying this style was especially difficult, and most of my energy was spent just getting it right. That, as well as the modding scene being brand new at the time of interest, led to the project not being finished, but I learned a lot about animation to complete the walk and style studying.
I've made a collection of custom sprites for the game Rain World by Video Cult using the popular community mod Dress My Slugcat. Player sprites in this game are separated into head, face, neck, body, arms, legs, tails, and anything extra on a character-to-character basis, such as the frills on the rightmost one shown here. All of these sprites have many variations for each angle they can be turned or action they can do in those angles, so it's a lot of trial and error when putting them all together in the game to look good.
For The Binding of Isaac: Rebirth by Edmund McMillen and co., I made a mod with simple code to add the correct hair and body sprites to the playable character Jacob and Esau.
A silly gif of a Goblin mech from the Lancer TTRPG rotoscoped onto a popular running skeleton gif.
These are a set of sprites I made to practice quick, simple asset production for a game jam that never went through. They intentionally have a messy style to practice work within a time limit.