I'm going with the Bulls...They have to prove themselves obviously but so do the Heat (who have the most pressure IMO), the Magic (after their trade earlier to shake up their team), the Hawks (always find ways to come up short in the playoffs).
The Bulls are a much different team than last years...they have a better bench and a better coach. Thibs is a better coach than Del Negro and more experienced. He helped the Celtics as an assistant win the title. He knows the Celtics well. If the Bulls get the top seed, watch out.