I'm a native (ex-pat) Montrealer and would suggest the only place to stay is the St. James. In QC, I love the Fairmont. There are more upscale boutique options like the St-Antoine (Panache is a great restaurant btw!) but I love the Fairmont for it's history. Great bar too (Le St Laurent). It feels like you're staying in a castle and the location is ideal. I prefer historic properties to modern luxury properties though. Only suggestion is to book a suite on the Gold Floors. It's a huge step up at this property in all respects. I haven't been in several years (since moving to California) but it used to be a nearly annual trip for me. QC is beautiful, especially in winter. Be sure to make a reservation at Le Patriarche. L'Echaude is a great bistro too. Le Lapin Sautee for brunch/lunch. There are tons of great patisseries and boulangeries all within walking distance like Au Palet d'Or, Paillard, Le Pain Gruel, J.A. Moison Epicerie, Choco-Musee Chocolatier, etc. Also, highly recommend taking one of the caleches outside the Fairmont to Loews le Concorde just outside of the old city walls one evening at dusk and taking the elevator all the way up to the rotating bar. Have a few glasses of wine or champagne and admire the view; after a few drinks you'll forget the room's moving and as you gaze out over the old city and views of the Saint Laurence River spinning around, you'll feel like you had ordered twice as many glasses!
Be sure to call a cab afterwards back to the old city for dinner. Whenever I'd visit, I would just stick within the old ramparts and cobble stone streets the entire time as it truly feels like a fairytale town.