in general, most and least third party friendly states

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Montana also has areas that elect third parties to the state legislature - sadly, it's the Constitution Party.

Yes, but the democrats stole that seat, so no worries.

Right, the Democrats stole it because they undid the result where the Republicans counted invalid votes (listed more than 1 person), just to tie it, and then broke the tie with the then Republican governor. Get your head out of your ass.

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Most would clearly be New York because of its election laws, allowing different parties to endorse the same candidate, and add the totals, and any party getting 50,000 votes in a governor election (even if it was endorsing some other party's candidate) gets ballot status for the next 4 years. 

In terms of statewide elections, a case could be made for MN, VT, and ME. However NY did elect a Conservative (that's a capital C) Senator in 1970.