Miami. I just wasn't a celtics fan back in the 80's or earlier, so the extent of the rivalries that I've witnessed as a fan are stronger and more recent against Miami. Miami's recent antics (the decision, "not 5, not 6, not 7", wade's punk acts, bosh's disgusting screams and faces, ray's free agent signing) since I've been a fan are far more loathsome as well. They've even managed to make me dislike Shane Battier.
But I expect that in 6 years, it will be the Lakers or perhaps some other team that has fostered a new rivalry with Boston nearing the level that Miami is at. I thought that level was previously unimaginable -- I think we're witnessing something in Miami that might be very unique in the history of basketball -- so it will very likely be Lakers.
Much respect to the traditional celts fans though who hate Lakers more no matter what.