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.
- The Templar is ideal for those who need a tank or require healing
- The Enchantress is ideal for those wanting crowd control and decent
- The Scoundrel is ideal for those wanting pure damage and a little CC
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
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
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.
Alternative Follower DPS Boosting Strategies
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.