Originally Posted by canadiancow

I can now confirm that in the "fare" part of upgrade priority, brand is more important than class.

Comfort S beats Flex M.
Premium Economy Flexible A beats Premium Economy Lowest O (this is easier to experience with Aeroplan bookings, but I have seen some interesting published cash fares that could replicate this).

I've updated the wiki.

Edit: I should also point out that this applies based on booked fare, regardless of confirmed cabin. If you're flying YWG-YYZ-LHR on a PY fare, confirmed in Y on YWG-YYZ, your upgrade priority is as if you were in PY, meaning you're top of the list.