Certainly wouldn't be touching a Navara, heard too many stories of dodgey gear boxes, $4000 worth of injectors being replaced after 90k, $2k alternators after 100k and the list goes on, the older ones may be worth looking at but nor the newer ones, which is a pity after the rep the Patrols have...
VW Amarok, suppose to be pretty damn good both on and off road and with the 8s auto could be worth a look but expect to be violated come service and parts time being a VW.
D-Max, awesome vehicle but cheap build quality inside and very basic, so if you are buying the 4x4 for its intended use could be worth a serious look.
Colorado, quite good and capable off road, however, being a Holden can lose resale value quite quickly.
Ranger, IMO looks the best and the 5cyl diesel from all reports is good, but I think with all cars if you search you will find bad reviews...
Hilux, the older ones are great (I have a 91 LN106) but they are getting dated and d4d diesel can have its issues and is getting slightly underpowered compared the other vehicles in the market, on the plus (or maybe its a negative) they do hold their resale value, interiors are pretty plain compared to some of the newer models around...