IMHO States like Arizona, Vermont and several others would be the easiest. They have conceal carry without permit sometimes called Constitutional Carry.
States like New Jersey, New York and others with may issue "justifiable need" requirement would be the most difficult.
My home state Madiganistan also known as Illinois will have most required training @ 16 hours, also offering no reciprocity with other states and an oppressive $150 for resident and $300 for non resident fee. Just my 2 cents.