A rather startling omission from the listings thus far:

Stan Rogers "Songs From Fogarty's Cove". I have heard David Woodhead reminisce that the nights they recorded those songs, they felt like they were participating in something really important, and they were: revitalizing Canadian folk music!

Looking at music of the Canadian Atlantic Provinces, there is so much great stuff to choose from in many sub-genres: singer-songwriters like Ron Hynes and Lennie Gallant, bands like Figgy Duff, the Barra McNeils and Rawlins Cross, and the great fiddlers of Cape Breton and PEI. But if I were to pick a "seminal" album, I think it would be

Alan Mills "Folk Songs of Newfoundland". This LP from the 50's contained the core of the Newfoundland repertoire. A few years ago a bunch of contemporary artists from the Rock released a CD of Newfoundland favourites which sounded like a contemporary remake of the Mills album--the songs were the same! As in, Kelligrews Soiree, Tickle Cove Pond, Jack Was Every Inch a Sailor, etc.

For an instrumental album from the region I would have to go with   
Natalie MacMaster "Road to the Isle". Her second recording, made at the tender age of 17. Fifty tunes, this is how a fiddle album should sound. Now re-released as part of a compilation of her first two recordings.