Definitely the WY Report, although the info isn't necessarily obscure aside from maybe crew first names and birthdays, and those tend to follow a predictable pattern. :p
The Tech Manual, due to licensing limitations, doesn't actually feature anything not in 'Aliens'. Like, the only reason the Nostromo and Narcissus get tech specs is because they're shown or mentioned in 'Aliens', and the only events from 'Alien' that get mentioned are those that Ripley mentioned in 'Aliens'.
The Tech Manual predates Prometheus by many years, so naturally it isn't featured.
I will say that the chapters in the WY Report kinda get progressively worse as the book goes on, and the Prometheus chapter (and to a lesser extent, the Alien chapter) is one of the best chapters. Things really nosedive in the Aliens chapter and kind of never recover, which is unfortunate.