I have a Bay State silver case size 18
what movement will be the best fit for that case?

Peter, About any of the earlier 18s movements should fit. I have Hampdens in Bay State cases and I believe I have an Elgin. Bay State was notorious for leading their gold cases and I try to avoid them if possible because to me they are not as well made as other makers, particularly the hinges. That may just be my luck though!

Thank you Deacon.