Mine is Lady Sia for GBA. It's just a platformer but I just love it played and completed more 20 times. Will probably speedrun it in future.
Seven Kingdoms II: The Fryhtan Wars
I found the disk at a Dollar Tree Store when I was a teen. I spent hours in endless matches trying to build my empire. I remember the spy system in this RTS was incredibly fun and nothing I've seen in any other game. Truly a gem with so much potential to become a cult classic with its charming art style.