After travelling to Cape York (via the OTL both ways) and then travelling around the Victorian High Country six months later, both times towing our camper trailer everywhere we went, I'd have to say that the High Country offers some of the toughest tracks I've ever driven on, and makes the famed OTL of Cape York pale in comparison. The High Country was tough not only on the vehicle but also on the skill and nerve of the driver, and I have never been so far outside my comfort zone when touring before, let alone when four wheel driving.
I've driven some other tracks which were not easy at the time, but more so because they were slow and time consuming. An example of these would be going into Bathurst Bay/Cape Melville via Starke Station years ago where it took 10 hrs to travel 70 kms, however most of this was washed out rutted tracks where 1st and 2nd gear was used most of the day, rather than four wheel drive. I also spent a very long night back when I had my 60 Series, getting into 5 Rocks Queensland, where around every corner was a bog hole, or sandy climb, or wash out track.