As a Montrealer, I really can't stress enough about trying a Smoked Meat sandwich at
Schwartz.
Even if somebody tells you that it's all the same thing if you go to Dunn's or Reubens, don't listen to them, Schwartz smoked meat really is in a league of it's own, it is different. Take it at least "medium" or "fat", just don't take lean.
However, Schwartz is a small restaurant, old and really not very comfy, but the meat is really worth it. I mean, try not to go at normal meal hours and you won't have to wait very long.
In general Montreal have a very strong restaurant scene.
If I had to suggest just a couple of places, "Au Pied de Cochon" and "Joe Beef" are two highly popular and praised by critics restaurants. Both are "quality without being snobbish". There is also the Toqué!, which is more about very high quality and fancy cuisine. More expensive, but a different experience.
You can buy the "Guide Resto Voir", which is a little magazine printed every year that is considered the "go-to" restaurant guide for Montreal. It should be very easy to find in any tourist spot.
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Watch this video :
http://www.dailymotion.com/video/xo2kqf_the-layover-s01e06-montreal_travel
It's a pretty good summary of good place to eat in Montreal.