First of all, make what like making and make what you are good at.
If you don't like a category of models, it will often be visible in the work in the form of a lack of creativity and inspiration.
And the better you are at making certain types of models, the faster you can produce them without compromising quality.
My best selling models the models i most enjoyed making and the models i had the best inspiration for.
But if you really must have other people tell you what to do, i would recommend using CG traders analytics system. that system tells you what models are most in demand (according to their algorithm).