I'm considering the Guide Archetype. Ranger's Focus seems like a slightly more versatile and powerful combination of Favoured Enemy + Quarry with the downside of being much more limited in uses.
However the problem that concerns me is how this impacts Master Hunter:
He can, as a standard action, make a single attack against a favored enemy at his full attack bonus. If the attack hits, the target takes damage normally and must make a Fortitude save or die. http://www.d20pfsrd.com/classes/core-classes/ranger#TOC-Master-Hunter-Ex-
Since Ranger's Focus replaces Favoured Enemy my hope would be that this feature of Master Hunter would continue to work on a Focused Enemy.