well, the only other competitor would be Eve, and i would consider it more difficult for a number of reasons: 1) Eve's atmosphere is very thick, and while this makes landing easier, it makes returning to Kerbin nearly impossible. 2) Eve has a much bigger difference in inclination relative to Kerbin than Duna. Duna and Kerbin's orbits are nearly co-planar 3) By the time the average user has mastered the Mun and Minmus, they will have only mastered going from a LKO to a target in orbit. Going to Eve, however, is going from one object in orbit to a lower object in orbit. The only way they would have had experience in this skill is if they practiced going from Minmus to the Mun.