It's a tie. And they're oldies, but great porn stands any test of time, no?
York Powers topping Cliff Parker in "River Patrol," 1995. Adam Film's World's Gay Movie of the Year, and still four stars at TLA. Deservedly.
I'd never seen Parker bottom before, and when I finally got this scene I knew if I never saw him bottom in anything else it was okay. One of those very rare instances in porn when the reality met and even exceeded the anticipation. River patrol head Powers catches underling Parker jerking off on duty, spanks the hell out of him (in mud!), drags him back to home base cabin, spanks some more, fucks the living hell of him, even threatens Parker's ass with a hot branding iron! The day this scene fails to lift my dick, somebody forgot to bury me.
York>Cliff
Doug Jeffries topping Dino DiMarco in 1999's "Sodom". Again, the first time I ever saw DiMarco bottom. Doug Jeffries, later an accomplished director, was a spastic jumping bean of a performer first, with a dirty motor mouth it would have been a crime to drown out in any film. He shows no mercy, pounding the near life out of DiMarco, while the semi-understandable utterances fly out of his mouth with no little amount of added spittle. Time will never damage either scene.
Doug>Dino