My most grueling experience as a passenger in recent years has been Auckland to London via LA. The flight sectors are long enough but what should be a break to stretch the legs etc in LAX is spent standing in a queue for American 'immigration' procedures. Even transit passengers are subject to that, even if you are getting back on the same plane.
Personally, I find the North America route Europe to NZ more tiring than the Asia route but I dont have an explanation for this.