What rifle would this caliber be best suited in? What are its effective ranges?
For what use?
Range- as far as you're able to shoot it. The 6.5's have excellent ballistics and are one of the best for long range target.
Look, it all comes down to the intended use of the rifle, and the range.
If it's target use, energy at any range is irrelevant.
If it's hunting, then energy has all the relevance...and the choice depends on the game, and the maximum range.