This I such a good thread, I will have to Google some of these teams, especially Michael Stefano and John Strong, who are longtime faves.
However the names I immediately thought of were the aforementioned Hakan Serbes and Nacho Vidal. Both men were at their peak; it was long before Hakan got fat & Nacho had the distracting tattoos and the murder charge. Anyway the demarcation lines between gay, straight and bi were very thick in those days, which made their chemistry in that Face Dance Obsession scene instantly popular. I had it on VHS and then bought the film again when it was released on DVD lol.
There are 2 other pairings that are worth mentioning. Tommy Pistol and James Deen have definite chemistry when they worked together.
However the most intense chemistry was between Karlo Karrera and Chris Strokes in the one of the Like Father, Like Son scenes from Devils Films. It was absolutely erotic.