Umm...no. Maine, Minnesota and Alaska.
Since the open primary states grant independents and third-party candidates equal and unlimited access to the ballot it is logically impossible to believe that any state could be more "friendly." That is, under the assumption the duopoly wouldn't pass laws to make it harder for their candidates to run than third-party or independent candidates.
You could say that in terms of the law, Washington, Louisiana, & California are the most third-party friendly states, but in terms of voter attitudes, Alaska, Maine, Minnesota, & Vermont are.
