I think the HoC's that get me the most are the ones written by JS devs who think 100% functionally. Actually a lot of the hard ones to grok were written by Jason Miller of the Preact fame, who seems to think out JS in a minified, functional fashion. I've had to step through and re-write several things just to see what was actually happening, then pare it back to it's original form before I "get it". It's super clever, but man did it take me a while to distill it's logic.