For ordinary, everyday grocery shopping, the large chains will usually suffice. They are dotted all over Montreal: Metro, Maxi, Provigo, IGA (which even offers online shopping) and even Loblaws, which is a recent invader from ROC (rest of Canada.) The food you'll find in these places will be remarkably similar, as they mostly buy from the same vendors or produce their own house-brand generics. Again, the most these places usually do is give ethnic food a nod with a small section perhaps called "Oriental products" down one aisle, where they will cluster all "Mexican" (dominated by the usual suspects, like Old El Paso et. al), "Chinese" (the usual bland plum sauces and Wong Wing westernized stuff) and perhaps some Patak's Indian sauces, with a few packaged Thai powdered curries or Kikkoman sauces, the rest of the shelf space being filled with Cup Noodles and other "Asian" snacks. Up until recently, it was difficult to find anything but long grain white rice in my local Metro, but now they actually put Basmati right there next to the Uncle Ben's. (Update 5/18/01: now there's Uncle Ben's Basmati!)
The vegetable sections are usually quite comprehensive, unless you like cardoons. There's usually a mix of local produce and imports, mainly from California. Even in the dead of winter (and it can get pretty arctic) you'll still be able to get broccoli, for example, although it may cost you $4 a head. The same goes for things like vine-ripened tomatoes and new potatoes--the pickings can get mighty slim come mid-January.
Meat sections are usually pretty good, but I've yet to see many of the chains actually include a live butcher on the premises. Everything will be pre-cut, so you can forget the sliced-to-order filet mignon or freshly ground chuck. Still, the meats and poultry are usually very reasonably priced, and if your tastes run to a little ground horse or calves' brains you may even find it next to the knackwurst.(Update 5/18/01: add Ostrich and Buffalo to the mixthings are starting to happen around heah!)
The bread in the chains is fairly generic. You'll find some fresh-baked in-house stuff like baguettes, but they usually get them from outside bakery chains. Their pastries are usually ordered from bulk-pastry houses.
Seafood is usually quite fresh, but I wouldn't be making sushi out of it. Cheeses are abundant, but are usually heavily weighted to a primarily French or Quebec selection. A wandering deli-type guy once gave me the hairy eyebrow when I asked him for Gorgonzola. "What's dat?" was his summary of my gourmet experience that day.
Don't look for your favorite scotch in the grocery chains, or in any grocery or other food store, for that matter. You will never find anything except beer and wine anywhere except the quasi-Stasi SAQ alcohol shops, where you will pay top dollar.