On paper, I'd say Montreal, Toronto, Ottawa, Boston, and Calgary are the biggest "pretenders".
MTL- just lacks the talent to have a real shot at the cup.
Ottawa/Boston/Calgary- same as above.
Leafs- lack experience, garbage D for first half-3/4th of the year
With all that said, I do think all said teams can surprise and make a small run. i.e. MTL off the back of price + their upgrade at coaching, Ottawa- Karlsson + Boucher's coaching + a lot of gritty guys who can play playoff style hockey, Boston- a lot of veterans with a lot of playoff experience, Leafs- a lot of youthful elite talent + Babs behind the bench, etc.
Basically, everyone has a chance in the playoffs, it's just about peaking at the right time. Even if it's a very small one.