Hottest sight so far has to be the look on my wife's face when a hung gentleman we know slid his cock in her for the first time...this was at a swinger's party, and she had been bent over the bed giving me a BJ. She had been eyeing his meat for a while and when he came up behind her and asked if he could fuck her, I think both of us (my wife and I) nearly came right on the spot.
One sight I'm dying to see is another man fucking her bareback....and then the sight of his spent cock sliding out of her pussy so that I can clean them both up. Not sure if I'll ever get to see that in reality, what with the health and pregnancy risk involved, but I'm hoping for the right situation to occur one of these days....