Modern Combat 5
This was my first complex reverse engineering project as it was a combination of MITM, dex decompilation and native binary decompilation.
I ended up finding out a lot of bad things about this game, like numerous vulnerabilities that allowed people to spoof messages everywhere, hijack in-game clans, view private chats and even personal data leaks. All of what I found I reported to the company that owns the game but nobody wanted to fix the issues much less even acknowledge their existence at all until I made a website that leveraged this research.
Overall their API really confusing with design choices that make absolutely zero sense but it was fun seeing issues getting fixed eventually.