Scott Burroughs' aerials (he did a summary), maybe the single best thread I've seen on the board.
The Honorable Company of Reverse Jans National Golfers, maybe the funniest thread I've seen on the board.
The Lawsonia aerial (my GCA home course).
The "what websites do you guys peruse" thread started I believe by Gary Daughters.
The (self-promotion...) thread on books that I started a few months ago, because it was both enlightening for the range of interests of GCA Discussion Board folks and it gave me a bunch of good ideas for books and authors to read.
I've been meaning to go back and bookmark a few more, including the fall Wisconsin Langford-Moreau tour and some of Dan Moore's course-specific (Culver, Kankakee) Langford threads.