Maybe some of the below criteria:
* outperformed their competitors, captured the most traffic
* created thousands of jobs, both direct, indirect
* achieved sustainable growth that has benefited the area it serves
No it's not. That Taj Mahal of a new terminal building cost twice what it should have, and that's why YYZ is the most expensive airport on the planet. In terms of "best managed" I wouldn't put it in the top five.
Never let the facts get in the way of a good story.
I'll tell you what one isn't: YYJ. The VAA has done some pretty silly things in the last little while. They just spend a whole boat load of money on 2 new bridges that can't be used at the same time as the exsisting two. When they redid all the lead in lines one of them had marks for 787-700 (which last time I checked wasn't offered by Boeing) and another was too far away from the building for our electric GPU to work (DHC-8)