I took the question as best arena atmosphere in the playoffs, because those are the fans of the other teams we see in the playoffs.
With that interpretation in mind, there is no question it's the Islanders and Nashville at #1 and 2, then Carolina at three with Vegas close behind. Panthers fans also put in a really good showing this playoffs I'm curious as to what other people used as criteria, because with the crowd atmosphere criteria, any answer that doesn't only include one of these five teams just feels...wrong.
There are lots of other teams that have good crowds. As a Sens fan in Ottawa, I've been to some of our playoff games, and the building was very loud. I still wouldn't put us up there with the top crowds. You can feel the hostility towards the other team and the excitement, enthusiasm, and support for their own radiating through the screen when watching best crowds. Most crowds don't seem as "intimidating" as the big boys.