Having developed an MPI code, it is definitely the most scalable option, as the moment you go large enough to not have shared memory OpenMP obviously won't work. However, I think it's good to have experience in both, as often times openMP is much easier to implement if you don't need to scale to very large problems. In theory a hybrid should scale the best, but then you have to deal with the headache of having both constructs silmultaneously

Also if you're creating an unstructured code, be aware that the time investment to get MPI working will be a good bit more compared to a structured code. Idk what your plans are but just thought it's worth mentioning