I want it to be us. There is a situation that plays out where we start clicking on all cylinders- perfect team defense and offense. In that situation we might be able to take them down.
But more likely it's the Bulls. Think about that roster. They are built to be a perfect opponent against Lebron.
Rondo hates Lebron. He plays big in big games and you know that he will be ready if they met in a playoff series. He can switch onto Lebron in stretches, and his passing helps them when their offense struggles.
Wade knows the Lebron. He is still an elite scorer. He can play him straight up if needed.
Butler is Lebron's stopper (sorry, Boris Diaw). Butler is big, quick, competitive, and forces Lebron to play defense too.
Gibson can play defense on Lebron in stretches, especially in the post. He can also rebound against Thompson and Love.
Lopez probably doesn't get a lot of minutes in a series against the Cavs, but he can bang with Thompson and foul Lebron hard at the rim.
McBuckets and Mirotic are the wild cards. If they give you elite shooting and consistent defense, then their top 7 are just as good as the Cavs top 7.
Bobby Portis could be really important too. If he can stay on the court (IQ and fouls), playing him allows them to stay big without sacrificing defensive versatility.
Also, Grant, Valentine, and MCW are all big guards who can play both ways if needed. I don't think these guys are probably ready for a playoff series, but they could have some big minutes.
If this team could stay healthy (big if with Wade), then they can upset the Cavs. I'd still lean towards the Cavs, but they are the first team in the East who I think has a legit shot. Imagine the following scenario: The Cavs decide they don't care about the regular season and the Celtics and Raptors take the first and second seed. Because of injuries and shooting droughts, the Bulls fall all the way to the 6th seed. First round, the Cavs play the Bulls in a gritty, physical, exhausting seven game series, but we get the Knicks and are able to dispose of them in 5 (just go with me on this!). If the Cavs did win, we would have an advantage for the first game or two.