Lots of correct answers. It depends a lot on what type of long range gun you after. Here's just a few questions that could change the answer:
Will it be for hunting?
If so, big game & coyotes or setting up over a prairie dog town?
Do you need to carry it for multiple miles a day?
Does it need to fit in a saddle scabbord?
What weight are you after?
What system do you want to use? Yardage direct dialing, MOA chart dialing, retical hold-over, etc.
Will you be faced with an occasional quick shot at close range in low light?
The scope that tops off a competition or primarily range rifle can be very different from what would be the best choice for a rifle that will be in the field, need to be carried longer distances, and be required to make hits under constantly changing conditions and time constraints.
The good (maybe bad) part is that once you build that dream gun, the next dream gun is usually already in the planning process!