I like notes that make you say, "Who the heck is that?"
Having seen the same dead presidents on my currency for over 50 years, I find it fascinating to see older notes that make me say "who the heck is that guy?" Some of them are downright obscure.
I also really enjoy the jarring effect of seeing a president on a note that doesn't belong there. For example, Andrew Jackson is on the 1914 $10 note...but he's always lived on our $20 note. That always shakes me up. "Hey, what's he doing on a ten?"