What is the best 'FREE' app to block spam calls on iPhone

I use the built-in Silence Unknown Callers option in Settings > Phone. I can't say it is or isn't the best but it is free and works perfectly for me. See: Detect and block spam phone calls - Apple Support

In life you get what you pay for. So any FREE app will be worth about what you pay for it. Your carrier probably has a spam-blocking app; usually it is free for basic blocking, but has a fee for anything more advanced. So start with that.

What Lawrence said.

Most providers, at least in the US, have turned on network features to help combat robo calls.

What features provided by your carrier would best be asked of your carrier.