The first moment my cock goes into the guys ass. Every time I am surprised by how good it feels. If you've been building up to it for a while, rimming, rubbing your cock on his hole, then the look of desire and pleasure on his face as you go in makes it even better. Followed really closely by the feeling of fucking a guy and knowing you're about to cum. The few seconds before, as the feeling builds up, you tell the bottom, deciding to stay inside, pushing deep...
As bottom, the best moment is on top, pushing down to get the full length of their cock inside me and feeling them cum inside me at the exact moment I shoot.