Domain, Subdomain, Subdirectory for 1 Niche in multiple states

Ok say for example my niche is social working and have a licensed social worker for every state.

Which would be better in ranking/seo:
a domain for each state?
a subdomain for each state while the main site be something like "socialworkingnetwork.com"
or subdirectorys to each state like "socialworkingnetwork.com/newyork

i would be trying to get each state on the first page of google from the terms like "social worker new york" "social worker chicago" etc...

which route would be the best to take?

thanks in advance for any help