I’d say the most useful thread as this: The Index of Elite Fourum Articles. It’s a list of all articles we have on the forum and almost all of them are worth a read imo. 
But some of my favorite threads in no particular order are:
- @pulpyfan 's collection thread
- The Great “Pokémon Hidden In Card Art” Thread
-
@shyguy 's collection thread (he surprised everyone when he made his collection thread. And I see he’s still active - and still shy regarding posting
) - @jkanly 's Master Guide for Pokémon WotC Corrected Error & Test Cards
- @pkmnflyingmaster 's “you are a member of the Elite Fourum” post
- E4 10th Anniversary Awards!
And I could probably continue quite a while. So many amazing threads and posts here on the forum.
EDIT: I see @trainerjack already mentioned the Index and Hidden In Card Art threads while I was searching for other links for this post. 
Greetz,
Quuador