mine would be entering easegill main drain at eureka junction, for the first time as a 18 year old in the early 70s , and i agree OFD the marble showers passage
The one that I have not found yet. But one of the most memorable in Yorkshire would either be KMC or strangely not sure if its classed as a master cave Red Moss Pot, only because I was not expecting something that long and big in there.