For me, nothing beats a challenging double apex. Deep Forest, the 3rd gear double apex right before the steep uphill section, is one of my favorites. The weird banking leading into the corner makes it hard to approach it at the right speed & angle. I can do it consistently, but often feel that it's sloppy. When I feel I did something sloppy, that's what causes me to improve my technique.
Another good one is the double apex immediately after the hairpin in SSR5, you have to carry a lot of speed through the hairpin in order to gracefully link the following double apex.