Coconut oil will freeze in the fridge, so no go. Unless you want to put it in tiny cubes, and take one out and thaw it before use.
Ghee is awesome - but will again solidify. But its awesome, and meant to be eaten as is. As Kids we used to wear it like lip gloss. Girls more than boys.
Pure Olive oil is bitter in mayo. That's why they mix it with canola oil or safflower. I actually don't care, I'd use it and deal with that little bitterness. Just IMHO.
MCT may be tasteless but in any significant quantity it could give you diarrhea. May not, unless you know you can tolerate it. But tasteless isn't neccesarily a good thing. Or is it ?
That leaves avocado oil as the best bet like Jey said. Just enough flavor, and stays liquid in fridge.

Thanks.
Srinath.