The Heat are obviously the most talented team in the NBA, so it's safe to say that they offer the Celtics their toughest test. However, if the New York Knicks are executing to the same level they have for most of this regular season, then they would also be favorites in any series with the Celtics.
Carmelo Anthony and Amare Stoudemire get most of the headlines in New York, for contrasting reasons, but the Knicks' greatest advantage in the playoffs will be their ability to stretch the Celtics' defense around the perimeter. The Knicks essentially start just one big man, with either Tyson Chandler or Stoudemire working inside, while the remainder of their offense is predicated on driving to the basket to set up outside three point attempts. From a sheer matchup point of view, the Celtics have enough guard and small forward depth to stick with the Knicks, but it is the quality of that depth that becomes the issue. Anthony would be the primary focus, and would likely draw the attention of Paul Pierce, while J.R. Smith coming from the bench would attract the interest of Avery Bradley. Smith and Anthony are the team's two primary scoring threats, but Jason Kidd and Steve Novak are pinpoint outside shooters, while Raymond Felton and Iman Shumpert have the potential to crack open defenses in multiple ways from the perimeter. On the interior, Kevin Garnett and Sullinger would need to match the physicality of Chandler and Stoudemire, something that would be incredibly difficult for the Boston duo.
The Indiana Pacers don't have the stars or experience of the Heat and Knicks, but they are arguably the most balanced team in the East. Crucially, the Pacers rank first in rebounds per game, with 45.8, primarily because of center Roy Hibbert, small forward Paul George and power forward David West all averaging over seven rebounds per game. Hibbert, George and West's ability to power their way past defenders would cause major matchup problems for the Celtics' interior defense. Add in the expected return of Danny Granger, who has missed all of this season through injury, and the Pacers would have too much size and athleticism for the Celtics to cope. The Celtics' defensive strength isn't about fighting back teams who want to overpower them, their defensive strength is about intelligence. Matching up to the Pacers' brute force isn't something they would be able to do. That interior power would force double teams and negate the defensive ability of Bradley and Lee on the perimeter. It would also make life more difficult for Barbosa when he is driving to the basket on the opposing end.
Even though they competed fearlessly against the Miami Heat last year, the Celtics' best opportunity to beat the Pacers would come from their huge championship advantage and post-season experience. The Pacers don't have the same track record of winning in recent years that this Celtics team has learned from.