Well, I have two most important mod's for me, and they both involve keeping the fluids up. Firstly, my ARB 47L fridge, for obvious reasons, but this also involves the second battery etc, so not really just a fridge mod. But this was my first fridge, having had to use Esky's before, and I would never ever ever go back. Its brilliant, and I am very very happy with the ARB fridge too over the Waeco my friends have for numerous reasons.
Second for me, was my 180L fuel tank (Custom made from the 145L tank from the 4X4 mob in adelaide I can't quite remember the name of..) I would still always carry a jerry can just incase you have a fuel tank problem, as in... big F*$K off hole or something, but it means I didnt have to carry three of four of them just to get anywhere. But that because I was living out northern WA and being able to do at least a 1000km between refuels is a necessity if you want to go anywhere. But it also allows me to pick the best priced fuel too, as I dont have to fill up as often if i don't want to. Down side is, I did have a few welding issues on the tank from when they made it that I had to get fixed to stop a few leaks, but overall, its awesome. And you do not loose your spare wheel space..... even with the 145L for standard D22's without body lift. It goes up and over driveshaft and exhaust. It was also cheaper than the Longranger 120L tank, made of steel (Fricken heavy, think mine was about 45 or 50kg, try putting that in your self.. haha)
For me they were the best mods, but thats cos I havent got to really use my diff autolokker out yet, and the rest I have done are good, but not as useful all the time, just when you need them.
....... Hmmmm.. cold beer.... hmmmmm