Worst documentation? Unreal Engine 4's documentation on editor customization (custom panels/windows and whatnot). It might have improved in the last two years, but the last time I made a custom editor there was almost zero documentation on the matter and on their Slate UI framework. The little documentation that existed was very vague and had awful examples.

I don't remember very well, but I think it took me close to two weeks to get something very basic working. I had to read a LOT of C++ code filled with generics and macros to figure everything out, but after I did I enjoyed a lot working with that stuff.

I just don't know how I was able to do that, working with UE4 was a pain the butt every. single. day. Runtime error on the gameplay code? Too bad, the whole editor will crash and then take ~40s to reopen. It was crash after crash, ~1min of compilation time for any little change to the code, so so so so much frustration.

I do miss a those times a bit though, because even though it was hard, it felt good to feel competent, to know something complex reasonably well to the point I could help people on forums. Today I always feel I don't know enough about the languages/frameworks I use. It's kinda depressing, it takes a huge toll on my self confidence. But whatever, let's keep going, one day I'll get there :)