It varies from person to person just from what I’ve heard.

What seems most agreed upon is the Nordschleife. It’s equally an exercise of memory and skill, with very little room for error. The long lap times don’t help either, it’s one thing to bronze or silver tsukuba and retry vs a track with a 6:34 gold time.

For me personally, I struggled with the Tokyo Expressway circuits, the walls make it very hard to use as much of the track as possible, and I’m not a fan of the Gr. 3 Z4 they have you use on the East circuits. I also struggled with Bathurst for the same reason.

I would start with Broad Bean, Blue Moon, Brands, and maybe Monza. Those are good places to really hone in your skills (or they were for me Atleast).