A headlock on Greg Jones at school when I was 10. Honestly it was immense I cranked that bastard 'till he went blue in the face.
In all seriousness the coolest submission I ever did was very early on. When I first started going to classes I would go with my friend I used to go to JJ and TKD when I was younger. After our first few BJJ classes we were hooked so used to start rolling ourselves after class. I pulled off a gogoplata from mount. Obviously our techniques were poor but I just copied what I saw from youtube.
To this day we still laugh about it. I have never managed to do it since though.
Chute Boxe Cabra!