re: Hardest and most interesting class you took in college

Most difficult as an Undergrad:Organic Chem or Celestial Navigation.
Most interesting as an Undergrad: There was a two part series that I took as a humanities elective: Patterns of Warfare to 1985 and Patterns of Warfare from 1865. The professor was a Marine Corps Lt Col (Gordon), who'd been a combat Lt in Vietnam. He was one of the best story tellers I've ever heard. His class was always full.

In Grad School,
Most Difficult: Community and Ecosystems Ecology. The Final exam took me from 8 Am in the morning until 7 PM that evening.
Most Interesting: Biogeography