I think Orlando will have the best chance (assuming Cleveland re-signs LeBron).
Stability is vastly underrated key in producing the familiarity needed to do well in the playoffs, and of the teams making changes, Orlando will have made the fewest to its core. Cleveland will have a new coach and potentially some new bigs, Miami will have an entirely different team, Chicago is looking at some big changes, and throughout all of this, Boston may be losing Allen and will be tinkering with its roster to replace Wallace, but Orlando is just upgrading its backup point guard job by signing Chris Duhon. Orlando is an elite-of-the-East team already, and they're not making major overhauls. I think that makes them the front-runner.