Best documentation have probably been most language docs and references I've worked with, official or otherwise, especially C++. Completeness, consistency, tidiness and examples really help a lot, since I know I can rely on the docs for basically any problem and makes work so much easier since I'll be guaranteed to leave understanding what's up.

Worst documentation has got to be the internal docs we had to create for a seven-man uni project, you couldn't find shit in the sea of docs that were out of date or just plain wrong. It was so much easier to ask whoever was working on that part about the intricacies of the cobbled-together mess than to either read the code or the docs. One absolute mouthbreather was working on the database docs and put in that it stored ArrayLists. Fucking Java ArrayLists in a motherfucking database. One day I am going to rant so hard about this dumbass and it's gonna be a spectacle.

Bonus points goes to the company's public documentation at my internship. It was good and pretty complete, but sometimes there was a document from 2 years ago that had been written by a non-english speaker that was absolutely awful. Some of them were so bad that as soon as I'd finished learning what I needed to, my mentor told me to go and fix the docs, I don't blame him.