1. Natural Freaking Spell
Blows Power Attack and everything else out of the water and technically isn't metamagic.
2. Power Attack
The king of (non-magical) damage dealing.
3. Rapid Shot
This is where things get tough, but Rapid Shot is a pretty powerful ability; you can net it at 1st level (as either a human or a fighter), it gives you an ability that's never available to melee characters outside of tightly controlled class features (flurry of blows, mostly), and it's actually better for a ranged character because ranged characters have less need to move more than 5 ft.
4. Combat Reflexes
Extra attacks are POWERFUL. Even conditional ones, like AoOs. Combat Reflexes gives a disproportionate benefit, potentially doubling, tripling or even quadrupling the number of AoOs a character can take. Especially good in concert with...
5. Improved Trip
The 'fiddliest' of the really good feats, Improved Trip gives you a) a bonus on something you'd otherwise have a hard time getting a bonus on and b) free AoOs - it rarely costs you an attack and usually gives you bonuses to hit. It doesn't create an infinite chain the way it was once thought to, but it's still an extremely powerful ability. The main reason it's weaker than, say, Combat Reflexes or Rapid Shot is because it pretty much doesn't work on larger, 'monstrous' type enemies.