In the past year or so, I've done YYZ-MEL a few times (AC, NZ and UA).
Price-wise (even with PE), UA worked out the best for me (to obtain 100% status miles), but it was painful to transfer through LAX (terminal change).
I didn't like NZ due to their arm rests (don't move all the way up if you have no one sitting next to you) and the timing wasn't as good (I try to sleep during the long segment, but that gets broken up due to the stop in AKL).
I didn't like AC due to the transfer to QF in Sydney - definitely not pleasant after finishing the long YVR-SYD segment. AC was also the most expensive (if you go for T+) with no miles on the SYD-MEL portion.
If I have to go again, I would likely go with UA.