Depends what you want and which companions (or mercenaries) whom you intend to use.
On higher difficulties, I believe that a bard’s competence bonus is extremely important and difficult to reproduce from another class. In addition, the revealed companions appear to be light on arcane casting. A bard can assist with the buff portion of arcane casting, leaving your other arcane caster to focus on save spells and damage. I prefer thundercaller as strictly superior to base bard.
If you’re interested in power over theme, I’d recommend strength slayer or vivisectionist over ranger/rogue. Both will preform better as DW, sneak attack damage dealer and vivi will cover some buffs as well.
Sorcerer is always good. As far as arcane casters, Ember is a stigmatized witch. I shied away from alpha, and don’t know her spell selection, but witches typically don’t have the best spells. If so, the companions seem light on an arcane caster.
Oops: thought this was a WOTR question. Companions obviously fill the bard and arcane caster roles in KM.