It has to be my masturbation after having watched my freshman college roommate masturbate.
He was a good high school athlete and had been voted the hottest guy in the dorms. He was very friendly, and very straight.
He didn't know I was watching -- or I don't think he did. I had been sleeping with my face facing the wall, but when I turned 180 degrees I saw his hand moving around his crotch. There was a nightstand between us that was tall enough for us not to see each other's heads when we slept.
I recall him lifting his head to see whether I was awake. I closed my eyes just in time. I gave it a few seconds and then opened my eyes. When I did, I saw him masturbating. He had pulled the covers all the way back, and had pushed his underwear halfway to his knees.
It was light enough for me to see the outline of his shaft and cock head. He'd lift his head occasionally and watch himself jerk off, and then drop his head again. He used his right hand and I was to the left of him, so I had a nice view. He used only one hand and would briefly touch his balls before returning to his shaft. His penis was a bit longer than mine, but not a lot. He probably measured around 5.5 inches.
He masturbated differently than I did, by holding the first two inches of his penis between his thumb and first two fingers. He was circumcised, but he seemed to use (what remained of) his foreskin in ways i had not thought of. He did not use his palm.
He'd speed up and slow down, then touch his balls, and then go back to his cock. This went on for a few minutes until he started manipulating his penis faster and faster. I will never forget when I saw him stop moving his penis and pressing it hard between his thumb and two fingers. I knew he was ejaculating, but unfortunately I could not see the semen spurt out. I did watch him lie there for 30 seconds or so, before rubbing his cum into his pubic hair. He had ejaculated pretty "normally" it seemed -- it seemed to land around his navel.
Then he pulled up his underwear, pulled up the covers, and turned his head toward the wall. He just let the semen dry, I guess. I found his underwear the next day, but was surprised not to see stains inside it.
Anyway, that night, I was more turned on that I had ever been in my life. I had just seen the hottest guy in the dorms jerk off, and I had knowledge of his hard-on size and the method he used to masturbate.
I pulled back the covers, pushed down my underwear, and used his masturbation method. I don't think he saw me because his face was facing the wall. But as you might imagine, it was my most-memorable orgasm because I had just seen a hot high school running back jerk off. He was the sort of guy I fantasized about in school.
This happened during the spring semester. It was the only time I saw him masturbate when we were freshmen. But we roomed together as sophomores too, and I saw him masturbate a few more times after he had pushed back the covers. I jerked off afterward each time of course and pushed back the covers as he had. At least once, I think he saw me, but he never said anything.
One time, he brought back a young woman to the dorm. They both must've been drunk. They got in bed and made out. Eventually, I saw him push back the covers and give her lessons on how to give him a handjob! He taught her his method! I'm sure I jerked off after that too, but with a female in the room, I didn't lie on top of my bed.