It's the "How Many Replies Can We Get?" thread, and it's been locked for about a year now; it was right up there in the Rumble Strip for some time, and had well over 100,000 replies.
Sounds like fun, right? However, it was really just a spam/chat thread, loading times took minutes even on a T1 connection, and lot of people lost interest when threats to delete the entire thread were proposed (as if their post counts weren't high enough already).
Right, now the "What are you listening to?" thread is one of the biggest post-counting threads, but "The Color Blue: An Argument" is far and away the spiritual sucessor to the old HMRCWG thread.