I have never had a three figure winner.
However last summer I took my 14 year old nieces to Perth. I asked them to independently pick a horse in the first(a maiden hurdle) but they managed to choose the same horse anyway. It was Just An Artist of Nick Alexander’s.
I thought it had no chance so I only put a few pounds each way. It led from the start, was never headed and came in at 125-1 
Our planned trip to Dynamic Earth in Edinburgh later in the week became a shopping trip to Princes Street instead as they spent their ill gotten gains.