Taking recommendations for what best favored enemies might be for a ranger....I am considering two things: most common enemies to get a lot of use out of it, and hardest enemies as well.

Human is obvious, but what else?

Animal is a good choice for favored enemies at the very beginning as well. They are pretty common in this game. As to hardest enemies, they might be: magical beasts, dragons etc.

Starter Ranger character has fey as favored enemies, but I haven't found any yet.

Yes you have. Mites are fey. So is redcaps.

Also there's a dryad to be found. I think she's a fey?

Human is good you fight lots of humans and they always scale.
I tend to go for thing that are rarer but reall tough Giants, monstrous hummanoins, abberations and or magical beasts...I don't do outsiders dragons or undead, since they tend to have innumities to "Crits" and the like.

Aazard, necromancer of long dead threads here:

The choice, "by the math" is clear:
1: "Beasts" are by far the most common in 5e and scale VERY well with PC's level & creatures CR increases
2: "Monstrosities" as a close 2nd in 5e, and also scale well with PC's level & creature CR increase
past this: and the numbers REALLY drop off here.
3rd: goes to "Undead"
4th: goes to "Constructs", "Fiends", "Aberrations", or "Plants" (Depending on campaign setting & DM's favorite critters)

Mostrous Humanoids
Undead
Magical Beasts
Fey

Won't let you down. There's at least one major dungeon for each, and usually a few related encounters.
Quite a few kobolds and goblins too, but those aren't terribly hard to kill.

The two common mobs that are most likely to cause you problems throughout the game are fey and undead (in that order) so those would be my first two choices.
There may be more "beasts" around but they usually don't pose much of a threat.

Yes, Fey, undead, constructs are the ones worth choosing. Magical beasts are plentiful, but not challenging enough to care. And everything else is one-off enough to spare instant enemy cast.

i hate this meta.
I favor gobelins as enemy.

I‘d say outsiders are even more problematic then undead. Although undead are probably more numerous.

Fey and constructs.

Hello, yes, this is irrelevant because this is about Pathfinder Kingmaker and not DnD 5e.