Tough call. Ravens are 6-2 and have swept the Steelers, but have two losses against subpar teams, and I'm still not sold on that offense. The defense, however, is still one of the best in the league. The Steelers are 6-3, with losses only to the Ravens (twice) and the Texans. The offense has been good this year, but for some reason, that defense just doesn't seem as dominant to me. The Steelers struggled against very poor teams early on (remember they only beat the Colts by 3), and I'm not completely sold yet that they've hit their stride (they did just lose). The Bengals are 6-2, with Dalton and Green being a great foundation for that offense, while the defense has really stepped up its game. It's easy to say that they've played a weak schedule (which they have), but they did beat the Bills and they did play the 49ers close. However, more will be learned when they play the Steelers and Ravens.
As of right now, I'll take the Ravens in the division, since they just seem like the most complete team right now out of the three. I doubt all three teams will make the playoffs (I feel someone in the AFC East has to take a wild card), so right now I'll say the Bengals make it, although my opinion could easily change if they don't beat Pittsburgh this weekend.