Biggest exploint? Nah…
Most cool one? Definetly.
Game: Commander Keen 4: Secret of the Oracle From 1991.
I discovered a secret area in the first level that gave you 8 extra lifes. Just take them. Die and rinse and repeat.
Note: I was at most 6-7 years old when discovered this myself and i rebought commander keen a few years ago, the secret area is still there!
edit: here’s a video of the area https://www.youtube.com/watch?v=Z29jQvVLaXY. Still blows my mind i discovered this on accident as a kid.
Tried putting it in rice?