My own thoughts:
Rocco Siffredi - Having been in the industry for so long and basically having done everything at least once, I think Rocco's biggest interest has always been in anal sex. Even though he's tried retiring in the past, I think the reason he keeps coming back is just cause other upcoming stars and directors just really don't have that drive to fuck ass.
Chris Strokes - From what I've seen on his Twitter and also based off his own personal website, Chris is pretty much an ass man.
Mike Adriano - Although Mike Adriano started off this way, I think he was pressured to make more pussy fucking stuff because anal only is still kind of niche.
Raul Cristian - Although he's long since retired, Raul really did have a way with how he directed anal. The Ass Traffic series were actually some of the earliest anal only movies I could find.
There are a lot of performers/directors that clearly prefer anal sex, although their movies are not anal only so that they please a bigger audience. (such a shame)
In their movies anal is the main event, the star of the show, and everything else is just foreplay leading to the main event.
Jean-Yves Le Castel is one of the pioneers of anal focused movies starting with the late 90s. He's Mr. Assman!
Christoph Clark from the same generation as Le Castel, one of the anal pioneers.
Before these guys came along anal was still taboo, maybe 2-3 minute in a 20 minutes scene was all that you could get.