Well for me...

I had a Queen and 9 of spades in my hand. The flop came up King 10 of spades and the Jack of spades came on fourth street. Jack had the ace of spades and not only bet into me with his flush but called my all in (at over 10 units)

He proudly showed he at the nut flush but then I flipped over my Queen and Nine of spades and said no I have the nut flush! - The Straight Flush! - it was an awesome moment. To have an impossible hand and to have someone pay you for it.