Most English speakers would say #2 sounds better, but they are the same. The difference is basically the placement of "not" because "don't" is a contraction of "do not". So the sentences could be: 1. I think it's not easy for you to conquer Mt. Everest. 2. I do not think it's easy for you to conquer Mt. Everest. You could also replace "not easy" with its opposite word, like difficult, ie "I think it would be difficult for you to conquer Mt. Everest.

It means the same thing. The 2nd one is more natural and what you would hear used commonly.