Unlike most contenders, the post (#31) by Triskadecamus in this thread is scarcely of epic length.
But it has stayed with me for nearly three years. I think that says something.
His Elephant Proof is also a classic.
Linking to the thread and indicating which post you mean like you did is also good (sometimes you want the thread for context). What is not good, in my humble opinion, is linking to a 8-page thread and expecting readers to find the needle in the haystack.
Flattered. Humbled.
I’m afraid it scrolled off the forum, but it was the thread that introduced the 1920’s-style death ray to the group. That whole thread was the reason I became a paid member.
There was a post that Satan once posted that was against the SDMB rules since it insulted another member. But even so, it had to have been the best, funniest, most clever post I’d ever read or seen.
I have no idea if it still exists and by your OP it wouldn’t really count anyway as it’s not in an active thread…but man was it great.
(Please note. Satan was way before my time. However I, for a long time, read page after page of old threads because I found them facinating. In the old thread (again, no idea if it exists or not still) when Satan was banned entitled “Unban Satan!” Arnold Wrinklereid (I THINK) is the one that posted a few of Satan’s warnings that he had garnered to merit a banning. One of these was the post in question.)
I know I’ve already had a vote on this topic, but I also have to mention zev_steinhardt’s post about his children
Appropriately perhaps, the only reason I ever saw this is because someone cited it in another thread as being a damn good post.
I found it! It isn’t a post as much as a thread. Sampiro has never been known (as far as I can tell) for being concise. But oh how he brings the funny.
Woman behold thy daughter, daughter thy mother, & BOTH OF YOU LOSE MY NUMBER!
I don’t know how many southern children I ended up emailing that link to.
On closer inspection, that post may have been what made me decide to join. It was posted in July of 2005 and I joined in August 2005. So, hey SDMB, Make sure Sampiro gets a royalty check. 
My post (see #36 above) is post #345 from that thread.