It's impossible to say that one hit "hardest," but his stories reminded me how my best mentors often gave me books to read on challenges I faced. So on my own dime, I bought a book on modern JavaScript for one of my folks to give to him - an area he's expressed interest in improving.

Even better is planned: I've mentored a young man for 10+ years, and he wants to learn to put it in with me. So he gets to do the heavy lifting, and we get time together. But that's for the spring...