My Ram allows me to customize my dash and choose from a bunch of different items. I find what I want depends on what I'm doing. While towing I really like the tranny temp. All the other fluids are nice as well so I often just tow with that on in place of my digital speedo. I'm usually not as concerned with my speed while towing anyway and can always just glance at the traditional gauge.
If I had to pick one I'd probably just go for tranny temp but any/all of the fluids are nice to keep an eye on.