[Favored enemy] should not be focused. [No +2 to a given creature type or somesuch.]
Instead it grants features that can alway be helpful.
Favored Enemy: Goblin
Perception +3
Sense Cheating: The ranger are immune to bonus damage granted from advantage
Dealdy Surpise: The ranger deals 1d6 bonus damage when he rolls an initiative highter than 10.
Favored Enemy: Kobold
Find/Remove Traps +3
Mook Slayer: The ranger deals 1d6 bonus damage when outnumbered
Hold Breath: The ranger can hold its breath for a number of rounds equal to four times its Constitution score before it risks drowning
Favored Enemy: Dragon
Insight +3
Fearless: The ranger has advantage against saving throws vs fear effects
Maw dodger: The ranger has advantage against saving throws vs breath attacks