For humans, at ranges of 100 yards or less (which is the most likely range and threat IMO), I want a good fragmenting bullet. Some will give you more range than that, but they are expensive.
For all purpose use, at longer ranges, for hunting large animals (i.e. Deer or bigger), and if your AR15 feeds them properly/reliably, expanding rounds like soft points or partitions might be preferable for the purpose of getting adequate penetration.
For me, I have XM193 as my primary stash ammo...