I’ll be flattered if someone puts one of mine on this list!!! 
Any of Master Wang-Ka’s story threads are certainly up for the award, as is the “Funny things said during intimate moments” thread and probably several of the threads that spawned SDMB cliches, like the “1920s-style death ray” thread–this was the first thread that really got me on to the board. Yes, it’s played-out now, but it was hilarious the first time.
“Guy Stuff” ran forever. I think it was approaching 1000 posts when it finally got locked down.
Scylla’s thread about being attacked by an inflatable toy blimp was so popular it began appearing on other message boards.
And sex threads are perennial favorites. We currently have fish feeding and commando waitresses collecting views by the hundreds.
The fish feeding thread is the funniest thread I’ve read.
The thread that satan started about whether or not he should let his girlfriend use a strap-on on him was a riot. Why satan thought it would be a good idea to bring this up on SDMB and ask our opinion remains the biggest mystery?
Nazi Groundhogs got me hooked on the dope and was started by scylla.
Now the OP is specifically asking for the most *interesting * thread. I’m not quite sure if the sex threads or funny threads should really be among the most interesting.
Can’t beleive no one has mentioned the Lord of the Rings one yet.
Please! :rolleyes:
The project we discussed for launching a goat into orbit from a PVC cannon was clearly the greatest.
Link? Please 
Here are a few of my favorites.
Preface from Czarcasm [from some other thread somewhere]:
How To Kill Evil Nazi Groundhogs
The Horror Of Blimps
Cervaise and the Telemarketer
The Monkey Butler Thread
Can We Induce Morality in SeaMonkeys?
Halvsie, the Two Legged Dog
Funny Things Said During Intimate Moments
Goat Porn
The Office Space Appreciation Thread
The TMI Thread
Guy Stuff
I just saw Star Wars…
Stuff The Youngsters Will Scratch Their Heads Over
The Goat Cannon Thread
Watershed Moments of the SDMB (contains links to many of these threads)
Classic Posts [from Fathom] (there may be repeats here too, I didn’t check first)
I forgot to qualify my list. As China Guy points out, the most interesting thread is not necessarily the funniest thread. My list above contains humorous threads.
The threads I’ve found most interesting might be completely boring to others. For example, the Exploding Cannonall thread, or the Pivotal Moments of WWII, etc. were fascinating to me because I learned a lot.
Thanks for the links.
I don’t even recall what the thread in general was about, but the most interesting post had to be the story of a poster’s classmate that would never take a shower in gym class. The boy managed to go quite a while skipping showers, but eventually the gym teacher forced him into the shower and pulled off his towel (or gym shorts?) revealing that ther boy didn’t have a penis. 
My all time favorite has to be , and I cannot think of the exact title, " What would the Onion do for the WTC headline"
It was so catharthic for me to participate in that thread.
Omigosh! I’m popular for the first time ever! Well, not me, per se, but at least I have something interesting to talk about! 
That thread has been more fun than I anticipated.
The TMI threads are way up there for me, though. Is there something wrong with me that I seem to be attracted to knowing way more than I should? 
Regardless of right or wrong, rest comfortably in the fact that you’re accompanied by teeming millions in this strange attraction.
Shirley Ujest, I can’t find the Onion thread you referred to. I wonder if it was lost in The Winter Of Our Missed Content.
My vote goes to auntie em’s Missing Coworker threads. A mystery inside a riddle inside a laugh riot:
Part One and Part Two
Where is auntie em these days, anyhow?
Possible Onion Headlines for 9/11
I couldn’t find it through the conventional search engine, but it was linked in another thread .