Having a bit of experiences with my small collection I would say people pay good for medical visualizations.
Things like how cells look/work or how bacteria infect cells, medical procedures, biological mechanisms in general, things that can be used for educational visualization.
For example how does photosynthesis work, how would you translate it into a 3D visualization, or how doe the longs manage to get oxygen out of the air and get it into the bloodstream or when blood coagulates what is going on there, how does that look up close etc.
In second place I would say creative ideas, unique designs and motion graphics stuff.
This is also the category I like to create for and luckily it also sells almost as good if not better then the medical stuff, so that one also gets my attention.
My real world stuff like the house, forklift, boat, violin etc, almost get no sales at all so I will avoid making that stuff.
According to CGtrader statistics animals is category that provides best value for effort, so if you like creating animals then that would be the category for you, it may be hard to do but then again easy stuff is easy so everyone can do it and no one sells anything or at extremely low price.