Without a doubt my Seido black belt test. I tested at 11 so the rest of the group were pretty much adults except for my friend Murphy. It was about 3 hours of heavy calisthenics, then every single technique from day one onward including katas and short form self defense techniques.
The of course Murphy and I passed, but as is tradition we could not get our belts till 16, so we got a nice black stripe on our brown belts for a few years.