LOL, I think the champ of all threads has to the the Hobbit Hole, which is into the multi 100k's of posts.
Over multiple threads. The initial Undead Thread established that the Last Post ("The End of the Internet") one can make on any particular thread is post #65534 (hexadecimal X'FFFE'). You can see it here
Amusing that FR reports it as 65544...
507 posted on 01/17/2007 9:35:47 AM PST by sionnsar
(†trad-anglican.faithweb.com†|Iran Azadi| 5yst3m 0wn3d - it's N0t Y0ur5 (SONY) | UN: Useless Nations)