I just went through all the forums. Here are the current threads which have the most posts:
Trivia Dominoes: Play Off the Last Bit of Trivia - Game Room - 4101 posts
Haiku Madness - Game Room - 1987 posts
The Celebrity Death Pool 2010 - Game Room - 1299 posts
Turkish flagged vessel attack [What if?–becomes What now?] - Great Debates - 1263 posts
The Reply Is A Song Lyric - Game Room - 964 posts
Fuck the Motherfucking Pope - BBQ Pit - 902 posts
Order of the Stick - Book 5 Discussion Thread - Cafe Society - 902 posts
What’s pissing you off today? [Spring Mini-Rants Can Really Hang You Up The Most] - BBQ Pit - 722 posts
Doctor Who Series Five: UK pace thread [edited title] - Cafe Society - 668 posts
The 100 Most Influential People - Try the Third - Game Room - 582 posts
The Great Ongoing Guitar Thread - Cafe Society - 568 posts
Get your baby’s dirty ass off the restaurant table! - BBQ Pit - 526 posts
LOST 6.17 “The End” - Cafe Society - 520 posts
2010 Postcard Exchange - MPSIMS - 465 posts
xtisme, come on down, you muppet! - BBQ Pit - 463 posts
Big Ten talking about being big 12 - Game Room - 413 posts
Helen Thomas Forcibly Retired - BBQ Pit - 407 posts
Is BP Doing Enough To Clean Up The Oil Spill? - Great Debates - 391 posts
Anyone feeling buyers remorse about Obama yet? - BBQ Pit - 386 posts
Too Proud to Die - the Fighting Ignorants - Game Room - 383 posts
The Ongoing Online Dating Advice Thread - In My Humble Opinion - 360 posts
Another “kid gets a hold of parents loaded gun” thread - BBQ Pit - 345 posts
You’re a fine example of a cop dude. Really. - BBQ Pit - 311 posts
Mom Busts Dad For 15 Year Old Custodial “Kidnap” – Now What? - Great Debates - 305 posts
When did religion jump the shark for great thinkers? - Great Debates - 297 posts
Why are the media ignoring the protest against the ground zero mosque? - BBQ Pit - 294 posts
2010 SDMB Chess Championship - Game Room - 287 posts
Has Any Famous Person Who “Came Out” Ever Truly Surprised You? - Cafe Society - 273 posts
2010 FIFA World Cup South Africa - Game Room - 273 posts
“Justified” - new FX series - anyone watch? - Cafe Society - 264 posts
Anyone want an ongoing gardening thread? - MPSIMS - 233 posts
Religion has given us nothing - Great Debates - 233 posts
The MMP is here - MPSIMS - 228 posts
What famous figures were you most surprised to discover were huge assholes? - In My Humble Opinion - 224 posts
Does every single thread about Africa need to get hijacked by racists? - BBQ Pit - 208 posts
Michael Moore is a Dishonest Hack - BBQ Pit - 207 posts
What’s your favorite lateral thinking puzzles? - Game Room - 206 posts
Who is the oldest celebrity that you are attracted to? - Cafe Society - 206 posts
NBA Playoffs! - Game Room - 203 posts