Pittsburgh. Without a doubt.
Their defense is outstanding, probably as dominant as the Steel Curtain. They probably have at least 2 future HOFers on that squad (Harrison, Palamalu) I heard all kinds of crap about how good the Ravens' D was. They didn't hold a candle to the Steelers. Fitzgerald may score, but Warner ain't gonna have time to wait for him to run 40 yards.
The Steelers offensive line has finally come together the last half of the season. Add to that Willie Parker being back and healthy, adds up to a good running game.
Roethlisberger is not quite it? What drugs are you on? He doesn't put up gaudy numbers like Brady or Manning(s), but that's because the offense isn't designed around the pass. He's an awesome quarterback. Shrugs off linebackers and makes plays (see Holmes' 65 TD score). He's accurate and can throw. The Ravens game would have been over at the half if the rookie Sweed hadn't dropped a perfect pass on the 10 yd line (thrown from the Ravens 40 by the way).
Here's Mike Wilbon's take on Roethlisberger:
http://www.washingtonpost.com/wp-dyn/content/article/2009/01/19/AR2009011900016.html
Sum it all up: An outstanding defense, a good running game that can control the clock and a quarterback that can make the deep pass and doesn't make mistakes equals a Super Bowl win.
One for the other thumb.