Sheesh...the BEST porn scene ever?!? Like asking what's the best band, best work of art, prettiest color... it's really all subjective, isn't it.
Many scenes I'd consider among the best were scenes in what is presently called "porn classics" (70-ish stuff) and in later amateur video porn (real amateur porn like the early "Homegrown Video" series - not the studio produced stuff designed to look amateur).
I wouldn't even know where to find clips of that stuff now, if they exist anywhere at all. And can't immediately think of any online clips that I'd consider remotely close.