See subject.
I’ve only been here three years, but:
Posts:
Finding an academic technical paper on optimizing the snack, crackle, and pop of breakfast cereals
Thread: (Tie)
A poetry thread, in GQ, no less, centering around moray eels
A thread on the odds of someone on earth physically seeing a neutrino
Sadly, one of the most interesting threads was the Umkay one… and it just got even more interesting after the whole big reveal that it was just a devotee who loved the attention pretending to be a quadriplegic. And the pitting and the Not Okay… it was all just like a big soap opera. But even still I feel like I learned a lot.
So many threads are interesting and I learn so much here, but I’d have to say that so far the most interesting one to me has been a revived zombie thread, about “Can you envision a water wheel and can you make it turn?”
Of course I canvassed the family. What blew me away was that the two who think spatially, who can make/fix anything, couldn’t do it. And the two of us dependent on words could. What’s up with that?
Unfortunately, my “best” posts are when I think it but refrain from saying anything. 
Best Thread: This one.
ETA: Best Post: This one (the after edit version, the original sucked)
Post: In defense of bodybuilding (I choose this because it prompted several PMs thanking me for standing up for something frequently ridiculed on here.)
Thread: The Infamous Umkay Thread
My finest hour was certainly the thread exploring whether crossing your legs was offensive. I don’t think I’ve ever made it two pages before!
It’s been almost fourteen years, man! I hardly pay enough attention to recall what I typed this morning.
It’s been a long time, but I really enjoyed participating in the story of a five-dollar bill thread. Shared fiction rocks. 
I had fun running this quasi-role playing exercise-might do it again once the holidays are over.
I’m not sure any of my posts really stand out.
I will never forget this post.
Favorite thread that comes to mind was “This turkey was killed by hand with a sharp knife”. Enjoyed posting, although I am no sharp knife.
Favorite thread-Any Botticelli I’ve been involved in.
I am most proud of identifying the discrepancy between intrade and other bookies about the US election. Sadly the thread was not especially well received.
Another great thread-malden’s Civil War “as it happened” thread.
My wittiest post was post #67 in this thread. The thread was about Governor Blagojevich being kicked out of office and replaced by Pat Quinn.
For those who don’t get the humor, it comes from a Manfred Mann song.
Sampiros threads come to mind. First of all, the one musical thread describing his car wreck in rural Alabama.