I haven't been through YYZ in ~3 years, but used to fly through there from US to Asia quite a bit before that. YYZ's a nice clean airport, and I've always preferred preclearance in Canada to, say, SFO or ORD. So it's a decent transit airport. I've never transited through any mainland Chinese airport after all these years, because I hear stories like RichardinSF's and so I've always avoided Chinese carriers. So to me, YYZ hands down. However, I much prefer YVR to YYZ. I don't know if YVR is an option for you. If you have a long layover like 3+hrs, I find YYZ's AC terminal to be boring due to the uninspiring food/shopping options in the international area and especially transborder. Would be moot if you're using Maple Leaf Lounge. Otherwise, YVR is much more interesting imo. Mind you, I haven't been through YYZ in a few years.