We may give a lot of opinions but it really is up to you to decide.
For me, its the D-Max. Good, reliable, fuel-efficient diesel engine with a wide torque band. The Ranger too is good but Ford isnt really known for its Diesel engines, Ford is known for producing gas-guzzling vehicles. Although the 380Nm of torque produced by the Ranger is something to think about.
The D-Max has a somewhat more relaxed style compared to the brute, alpha-male structure of the Ranger. Depends really on what type of person you are. The Ford is tough-looking while the Isuzu is a little bit sedate.
Price notwithstanding, D-Max is the best buy.
How should I know about this?
Well, we have a D-Max ourselves..
Good luck though on your purchase and have fun with it.
Choose wisely as to not regret it in the future.