For me it's TIM's Loaded featuring Dawson. I stumbled up a clip of it when I was maybe 12-13 after searching for gay porn online, and after tracking down the full video later, I'm convinced it changed my life.
Watching one man after another blow their load into one hole, using the cumulative cum as lube got me hooked immediately. After continuously watching that (and other Dawnson gangbang vids), I became obsessed with barebacking, anonymous sex, and group play.
Almost 20 years later, my dream is still to be the very last top in a similar scenario, eating out the previous 20-30 loads, and using it as lube to add my own. Maybe one day...