I think the 2015-2016 Celtics, Hawks, and Raptors were all about on the same level, though you could certainly argue that those two were just a bit better overall than us.
However, I think the addition of Horford puts us CLEARLY as a better team than both of them and firmly in the number two spot in the East. Both of those teams had much more talent than us this past year, but our chemistry, fit, and playing style was so good together that it raised our collective game to be more than just the sum of the individual parts. By adding another excellent-fitting piece in Horford, we definitely are the better team than either of them. And I definitely think that we're the best matchup against Cleveland. We know the Cavs are the Hawks kryptonite, losing something like 12 straight games to them, and we know the Raptors simply don't have the team to compete with them. Cleveland is clearly better, but I definitely think we'd make a close series of it with the addition of Horford.
Let's also not forget that pretty much that same Hawks team won 60 games the year before.