The best BMI (body mass index) to be a model today will depend on a variety of factors, including the type of modeling you're doing and the market you're working in. In general, fashion models are expected to have a very low BMI, as they are often required to wear clothing from designers who use very thin models to showcase their collections. The average BMI for a fashion model is around 18.5-19.5, which is considered to be underweight. However, it's important to keep in mind that there is no "perfect" BMI for a model, and that different markets and clients may have different preferences and requirements. For example, some clients may be looking for models with a more athletic or curvy build, and may be less concerned with the model's BMI.
Overall, the best BMI to be a model today will depend on the type of modeling you're doing and the market you're working in. It's important to focus on maintaining a healthy weight and body composition, rather than trying to achieve a specific BMI number.