The hottest that I saw was when my wife and I invited a friend of mine over with the plan that she would try to seduce him. We planned that I would leave to go get a pizza and she would try to seduce him while I was gone. The agreement was that I'd turn off the porch light when I left, and when she either had succeeded or given up, she'd turn it back on, save I'd stay gone until she did so.
Unbeknownst to either of them, I just drove down the street and walked back, then positioned myself outside the big picture window in the living room. I was hidden behind a shrub from view of anyone passing by, but a bit to my chagrin, with the porch light off, everything they were doing could be seen be anyone who might happen to look that way while passing by. Oh well, I was too excited to care.
I watched as she brought out a photo album, and used the pretense of showing it to him to position her big breasts at his eye level, her arm draped around his back. In time she placed her hand suggestively on the back of his neck, stroking it gently. He responded by putting an arm around her waist and then letting his hand wander, casually at first, then brazenly cupping her ass, and sliding a hand up under the back of her top.
She encouraged him further by reaching back and unhooking her bra for him. He took the bait, lifting her top up exposing her breasts, taking them first in his hands and then in his mouth.
Because his back was to me, I couldn't directly see all of what happened next, which oddly made it even more erotic.
I could pretty well tell that her hands were playing around with his cock but couldn't tell exactly what was happening. Soon the positioning and movements were enough to convince me that she was undoing his belt and playing with his dick for sure. Then the unmistakeable movement of him helping her by pulling down his pants. Her hand was clearly pumping his dick now, and then I thought I'd blow my load a I saw her drop to her knees and there could be no doubt but that was sucking him.
I had to tear myself away from this sight, but didn't want it to be obvious that I was in in it so I had to go get the pizza. When I got back, the porch light was on. I entered as though I knew nothing.
My wife excitedly pulled me aside at the first opportunity and nervously whispered to me that she had sucked his cock. So hot.
After the pizza and several beers, we finally got something going and i got to see his cock going in and out of her pussy. Which was great. No denying. But nothing was as hot as watching her suck him while neither of them knew I was watching. I never told her.