I'm of the belief that Thomas will look better and Love should be healthy by the playoffs which still makes the Cavs the biggest threat. The Raptors would likely be favored in a series against the Celtics as well. Frankly I wouldn't be overly surprised if a team like the Bucks, Sixers, or Pistons beat Boston in a series in the first round. All of them have players that are difficult matchups for the C's.
I would be stunned beyond belief if the 76ers or the Pistons beat the Celtics in the first round. We are 39-15 and the 76ers and Pistons are both struggling around .500. I understand that the Pistons just added Griffin, but that team has zero perimeter shooting and their depth is horrendous.
As far as the Bucks, yes, they are tough because of Giannis and their length, but let's remember that we have had their number so far this season and that they just had to fire their head coach because they have been so disappointing.
Barring injury, there is no reason why the Celtics should even go to a Game 7 with any of those teams; let alone lose a series to any of them.
As far as the original question, the obvious answer is the Cavaliers. They still have the best player in the world, and we need to give Isaiah Thomas a chance. He has only been back for a month from a serious injury with an entirely new roster which plays an entirely different style of basketball from what he is accustomed to.
Even if IT isn't able to completely integrate himself into Cleveland's system, again, the Cavs still have LeBron, and that may very well end up being enough against any Eastern Conference team.