Most useful: ethics. I’m shocked at how many unethical people I continue to encounter on a regular basis. Also, portfolio management. I work as a portfolio manager and asset allocator and you’d be surprised at how often investment professionals lose sight of the forest through the trees. Particularly security analysts.
Least useful: specialized accounting topics. Like pension accounting doesn’t make much economic sense and continually undergoes change so difficult to master if not actively involved in the craft. Inter-company accounting and capitalizing operating leases (before GAAP rules changed requiring capitalization I believe). And derivatives, although this is a subjective response. My first job out of college was spent on a derivatives structuring desk managing overlay portfolios for institutional clients. I also studied derivative pricing extensively while completing a MBA at Booth. The CFA curriculums’ treatment of derivative pricing is pretty amateur from a practitioner’s perspective.