I suppose it depends upon what you like ultimately.

If by diffusers you mean those bottles with sticks stickking out of them, most essential oils are not thin enough to work with those. When you see them in stores, they are generally filled with synthetic fragrances that are thin enough, and a synthetic carrier (although I don'tthink they need one per se)

Olive oil could be used for a base, but generally only for very strongly scented essentials. Cinnamon comes to mind.

a common one is almond oil. It has a very neutral scent that is easily covered by other scents.

Grapeseed, Fractinated Coconut oil, really anything, However, with most oils, there is a caveat. Oils do go bad. I cannot remember the specific word for the life of me right now, but basically, they rot. This can be held off with a good preservative. A common one, that comes recommended is Vitamin E added directly to it. A gallon of any regular oils with a good tablespoon of vitamin E will preserve it for years. Smaller volumes need less Vitamin E of course.

Another option is Jojoba, which while a liquid, is actually a wax, not an oil. Wax's don't go rancid (that's the word) and so can last a very long time. It is very popular for that reason.

I personally use a mix of my own concoction for the odor, absorbtion rate, greasiness factor and preservation quality. Experiment and find what you like or think what works best.