I hear repeatedly that some flights are more likely to have upgrade space available than others. But I've never seen any specific mentions of which flights those would be.

Off the top of my head across the atlantic there are LHR flights to YUL, YYZ, YOW, YYC, and YHZ. From CDG there's at last YUL and YYZ and probably some others.

The impression I had from flying LHR-YUL was that there were already a long queue of people waiting praying for their upgrades to clear. Often several of them were called up but it seemed there were a lot more disappointments. Is it better to connect in YYZ or YOW? Is some of the a/c or times of day to YYZ more likely to have space? Are the flights from CDG different?

Are there flights I'm not considering that are a better bet? GVA?