Hard to argue against TMNT since it is considered the best game for Gameboy. I remember playing it as a kid and I actually think it was the first game I was consumed by.
I just read GTA 6 is also coming? Reference: https://www.gamerevolution.com/news/402387-report-gta-6-release-date-teased-by-online-message https://www.gamesradar.com/gta-6-release-date-news-trailers-map-gameplay-rumours/ also GTA 6 Release Date
Hands down Pokemon and Donkey Kong, i would spend hours and hours on those and at some point i was even bringing the game boy with me at school