I know there will be a lot of unhappy artists telling me to shut up but the best categories usually tend to be 3D models related to interior design. Those tend to sell better than cars 3D women and a bunch of other models that have been done to death. Again though it all depends on how lucky you are because even when you make great models they don't always get seen by the buyers you want to sell them to. The best way to go about making models is to create them for existing clients. Then you get paid for making them and then get paid again if they sell. There are very few people interested in buying 3D models and the ones that do usually have their favorite clients that they like buying from. That is why getting noticed is very difficult.