Not a Teacher.
I believe the second one is correct.
Both sentences are told in past tense, and both say that he didn't have to overcome the obstacle. The very start: "It would have been..." tells us this.
However if he did have to overcome the obstacle, it would have been one of the hardest obstacles.
The word 'would' in this case makes overcoming the obstacle seem hypothetical and that he didn't have to do it.
Student or Learner