My best thread was The have you ever thread, which almost went over 1000 considering it was just stupid questions (sometimes funny though). Then it was locked with no explanation and I think toastboy or someone else made another have you ever thread which went to a 1000 post or more...
http://gamrconnect.vgchartz.com/thread.php?id=69310&page=1
It wasn't really legendary but it kept the forums fun when not much news going on