Working east to west....
LHR T3 - One of the pubs there - can't remember the name - if you can get a seat does a semi-decent cheeseburger otherwise the place is a dead loss - I mean ugly.
YHZ - nothing decent in the airport. Backwoods. The sticks.
YUL - semi-decent restaurant in the check-in area. That's about it. Sad lot there especially for such a large cosmopolitan city.
YOW - really great small deli - fabulous smoked meat sandwiches with kosher pickles and fries. A treat. Get one to go as well.
YYZ - Can never remember any thing decent to say about the place, restaurants, bars, lay-out, most staff or anything else. There's a reason(s) for that.
YYC - Western type (Yee-Haw type) place does a good all-day breakfast and has a nice bar. Harvey's does a good burger near the A gates. Tim Hortons' does fairly good sandwiches but their muffins are partially-cooked somewhere in Ontario and finished at various places across the country - and this is a big country. Plus about half a dozen mediocres on the food level.
YVR - Tons of restaurants. Some good (Milestones in domestic) (Whitespot on pier C) ( Harvey's/Chalet Chicken downstairs in Domestic) - some mediocre, A+W, Burger King, Pizza place, Sushi place, Chinese noodle place, Japanese restaurant and more. Lots of mediocre to choose from (Tim Hortons included).
This and other reasons are why I always travel with a couple of Bacon Sarnies east or west bound.