I bought 2 Mitsubishi dual cab 4x4 utes for my small business about 6 months ago (after test driving the various choices) and they are reliable, excellent value for money and... dated in styling and layout. If you're buying new and your budget is in the low $30,000 range then it's hard to beat Mitsubishi's deals at the moment however I'd strongly suggest one of the more recently released utes if your budget will allow it.
The Nissan Navara D40 is if anything more dated than the Triton (and not as well priced) so I'd cross that off the list as well.
The Toyota Hilux has the name, the reputation, the best dealer network and the greatest availability of after market accessories but the current model isn't far from replacement. That can be good (you'll pay a lot less now than when the new model comes out) but also bad (you'll take a depreciation hit when the new model comes out and you'll be looking at all the new shapes and wishing you had one instead). So Hilux? Sure if you're a Toyota man through and through but otherwise I'd look elsewhere.
The Ford Ranger (or it's oddly styled Mazda twin) would be my first choice followed at some distance by the Holden Colorado.