I don't know about toughest, these teams the Knicks I think will have to prepare for the most:
Lakers - With the addition of the best passer in the league, and a legendary all-around shooter, they have arguably the best backcourt in the league, if I was guessing, they will be. They also have a very, very tough front-court in Bynum and Gasol, at least when they're on, which they weren't against the Thunder in the Playoffs this year. We took care of a lot of that with the addition to our team with Chandler, and now we're ready for the set-ups with Nash with the addition of Camby, Kurt Thomas, and Kidd. Shumpert will also be big when he returns.
Heat - Ray Allen may not be playing at a superstar level anymore, but on this team, I wouldn't be surprised if he has an impact of one. Battier and Miller got hot in the Finals, Ray Allen being hot is the norm. Who knows how Wade will play in the regular and post-season next year. On the decline or will he be back to being a superstar?
Those might be the two toughest. The Thunder has given us problems the last few seasons, but now we have a lot of defense and instead of D'Antoni we have Woodson.