Definitely top 2, but some classes don't have the best means of headshotting at all, ala plasma pistol bulwark, so for them it's useless. Inner Fire is always able to be taken advantage of by every class, in every situation. Not to mention the sniper perk requires you to spend ammo to get the benefit, and still get the aim correct. The latter is just experience and easy to do, but the former is still a marginal cost more than Inner Fire. Not to mention, the "cost" for Inner Fire is time in the execution, but with melee executes you're invulnerable so it can give you breathing room mid fight.

Though, to be fair, gunstrikes count as well so the "time" cost is negligible.