The reason finish is applied to barbells is to prevent rust. The choice of finish will also impact the feel and grip of the barbell on your hands. Bare steel bars require continual maintenance though they are typically the most cost effective option you can find. Black oxide finish is the next most economical option, but it offers only a slight improvement in rust prevention vs bare steel because it is only a fine oxide layer which can be easily scratched which allows rust to settle in and spread. Zinc coating is a popular option for the excellent combination of rust protection and grip. Zinc has better rust protection than oxide and cerakote alternatives without substantially impacting grip. Stainless steel is the most expensive option and offers the highest corrosion resistance and maintains a feel similar to bare steel.