Game Breakers (780 Baseline Road) has the largest selection of singles by far, but most of it is stored in a back room and there is very little actually on display in the store. Great if you going armed with a want list; not so much if you are just going to browse.
Miraj Trading (1564 Carling Avenue) has the largest selection of unopened product, including numerous hockey boxes going back to the early 2000s. Not a lot of singles but a few quarter boxes and dollar boxes that are okay to flip through.
Sports Card Corner (716 St Joseph Blvd in Gatineau) has the largest selection of hand-collated complete sets I've ever seen anywhere (all hockey, of course). They don't have nearly as many singles as Game Breakers, but what they have is much better organized and easier to browse.
Out of the Box Collectables (1826 Robertson Road) has more unopened product than Game Breakers but a whole lot less than Miraj. But their dollar boxes are usually a whole lot better than the dollar boxes at Miraj.
Neither Miraj nor Out of the Box do any vintage at all, in case you would be looking for older stuff.