I've tried playing with all three followers, and even with pretty good equipment they only manage to achieve a small fraction of the DPS I can dish out.
What is best follower configuration for maximum damage output? Specifically:
I'm also curious if there is a different answer for AoE damage vs. single-target damage.
I'm going to refer back quite a bit to what was said in the "What are the strengths and weaknesses of each follower?" question in this answer.
So taking that the Scoundrel is your best chance for direct damage, your first step is to gear him properly with as much Dexterity increasing gear as you can to maximize the 2.5x multiplier.
For his skills:
Level 10 - Poison Bolts
Level 15 - Personal Preference
Level 20 - Multishot is better for AoE mobs, Powered shot better for boss fights
Level 25 - This will depend on your current class and its focus on critical hits. Hysteria if you do have a high crit rate, Anatomy if you don't.
Direct DPS from your follower is not always the best strategy to follow, you might want to consider some alternatives of how you use your follower.
A good alternate path to follow is to pick the follower that can give YOU the most benefit and increase YOUR DPS. In that case, I like the Templar with his level 20 Inspire ability, particular as a melee class. It allows me to bring a more consistent stream of Spirit or Fury spending tasks, and since I can provide MUCH more DPS than a follower, i think that is the best course of action.
Another benefit you get with the Templar is that since he is a tank, and will charge headlong into combat, you can give him lots of indirect DPS through items that contain "Thorns" damage. Some rings, amulets, and shield will possess this damage, and at high levels they can possess enough to likely outpace the follower weapon damage output. If you decide to gear your Templar this way, be sure to give him items with the highest armor and strength/vitality possible so he can survive the pounding.
If you ask me, most of the benefits of the Followers aren't their DPS, rather, it's their utility; their damage seems fairly lackluster in general. That said, the Scoundrel looks to have the most DPS-oriented skill choices, so if you're dead-set on maximizing your Follower's DPS, I'd try choosing the Scoundrel and then outfit him with the best gear you can find for him (Dexterity/Damage, etc.), and the following skills:
Level 5: Poison Bolts
Level 10: (Doesn't matter)
Level 15: Multishot, though I'd guess Powered Shot may be good in some situations
Level 20: Hysteria (again, Anatomy may also be good depending on your class and the situation, though)
The Enchantress has some decent damage too, I believe, but most of her abilities seem focused on utility and buffs rather than outright damage, so I'm not sure how well she'd work out from a "maximum theoretical DPS" perspective.
Followers get 2.5x benefit from stat boosting affixes, so you need to load them up with items that increase their primary stat (eg. STR for Templar). Since Templar can have bonus on 1 extra item (he has 1h weapon & shield, instead of 2h for scoundrel or enchantress) he should have an advantage in being able to get more bonus this way.
Good point about AoE damage, Eirena might win that one.
