Adopted Ancestry (1). There's a bit of a feat tax to this one but it does open up some options for characters. A good example of this is getting the goblin's Burn It! and Junk Tinkerer feats or the dreaded Gnome Weapon Familiarity (or Unconventional Weaponry) for the Flickmace.

Ancestral Paragon (3). Some Ancestries have great feats, it's worth knowing about this.

Ancestry Feats:

Halfling (Cultural Adaptability) - You have to wait until level 5 but this removes the feat tax from Adopted Ancestry.

Human (Natural Ambition) - A second level one class feat is great for certain classes -- particularly alchemists.

Burn It (Goblin)

Animal Accomplice (Gnome)

Incredible Initiative. It's pretty hard to come by bonuses to initiative rolls and going earlier in combat is almost always better.

Fleet. Increase your speed by 5, good on everyone (maybe takes a slight decrease for characters relying on mounts).

Untrained Improvisation. Probably the best general feat in the game and about as close to a must-have for characters that aren't skill monkeys. The only knock I have with this feat is that it's table dependent -- one of the groups I play with generally don't allow rolls if you're untrained in the skill, so I'd assume this feat would be of zero value at that table.

The weakest feats? Traps?

Armor Training. This doesn't scale with your armor proficiency but will be good for Wizards until they are level 13 then retrain out of this.

Canny Acumen. Every class has a weak saving throw, this helps you get around it. More important for the classes that have poor Perception progression.

Toughness. This effectively gives you a +1 bonus when calculating your hit points. Useful but kind of underwhelming -- you're better off increasing your CON with each skill increase. More important for Ancestry Flaws in Constitution.