From my experience the Club Desjardins is the worst place for ambiance. However you have a nice view of the game and the crowd. Red B is nice for the ambiance but you're far from the action when it's in the other territory. If you plan to go more than once the best is to try different areas, then you'll find what you like the most for next year
Hopefully in the Desjardins section you're in the middle of the row, because people are always standing up to find some food. Like, always.
They don't care if it's during play or not.
Molson Zone is by far the best place to enjoy the atmosphere of a Habs' game, especially during playoffs. True fans are right there.
In terms of views? Maybe it's just because I have great eyes as an 18 year old, but I always have a clear sight at the game regardless of where I am sitting. However, like others have stated, if you want to be around other die hards, there are certainly some areas in the arena that you don't want to sit in.
I went to the game vs the Avs earlier this year and I was on the last row, right next to Serge Savard's painting. I had a pretty good view, but I couldn't see the big screen. That was the only bad point.
In my opinion, there are a couple of sections I don't like to be in:
Grey and blue in front of the press box because you can't see the jumbotron.
Grey and blue seats right behind each goals. When the puck is behind the net, it's pretty hard to see anything because everybody leans forward.