For me its the Heat and its not even close....
I just cant stand Miami ever since Pat the Rat left us for them, which was the begining of one of the best rivalries in my time watching the Knicks and the NBA. Add the fact that LeBron chose to go there over NY, not to mention team up with Wade and Bosh talking about winning 7 rings before they put on a uniform, and there is just no other team I hate more than Miami.
Growing up I hated Chicago but had more respect for them than I did for say Miami or Indiana as they were just that damn good, even though they broke my heart by knocking us out of the playoffs year after year.
As for Boston, I have never had anything against them. Probably because when we were good they sucked, and when they were good we sucked so the rivalry was never there (in my lifetime atleast). Dont get me wrong, I HATE THE PATS as a Jets fan, but the Celtics have never bothered me, that may change though especially if they knock us out of the playoffs this year if we face them.