Hi Brooke,

Whether you are in Portland, Maine or Portland Oregon the breeds you mentioned below would all do well with the weather.  So lets take it step by step as there are a few more pieces of information that would make this more helpful.

1. Will this new addition live inside or outside the home?

2. Do you have a fenced in property and if so how tall and strong is your fence?

3. Are you more interested in protection of the home or of yourself while hiking?

4. What terrain and distance do you normally hike?

5. Have you had any experience with a very large powerful dog like the CO or Sarplaninac?

All things being well and based on your description so far I would recommend the Great Pyrenees or Sarplaninac. Both of these breeds will give you headaches with hair inside  your house if they are housed inside but will provide the necessary deterrence to an intruder and are imposing enough to keep bad people away just by their stature and size. The Shar - is heavier duty guard dog of person, home and property than the Pyr.  Notably left off my recommendation are the CO - maybe to hard of a dog for what you need and when actively defending they are very very motivated and difficult to control.

The Newfie, St. Bernard, and Leo are not close to what you describe as needing.  I would add the American Bulldog and the Cane Corso to your list - and give them a good look also. 

Lets continue the discussion as others jump in.