Which monk skill will do the most damage to a single target? [duplicate]

You'll want to look at the best spirit generator, and the best spirit spender, since you are concerned with overall DPS. Spirit spenders will give you more damage per attack, but you need spirit to use them, so you need some sort of rotation.

High-Damage Spirit Generators:

• Fists of Thunder (Bounding Light) does 110% damage + 73% damage every 3rd hit
• Deadly Reach (Scattered Blows) does 110% damage + 170% damage every 3rd hit
• Deadly Reach (Foresight) does 110% damage, but increases all your damage by 18%
• Crippling Wave (Mangle) does 143% damage, and has some nice debuffs
• Crippling Wave (Breaking Wave) does 110% damage, has nice debuffs, and makes targets take 10% more damage from everyone
• Way of the Hundred Fists (Blazing Fists) does 140% damage, & increases attack speed by 15%
• Way of the Hundred Fists (Fists of Fury) does 140% damage, + 10% damage per second ongoing
• Way of the Hundred Fists (Windforce Flurry) does 140% damage, + 250% damage every 3rd hit
• Way of the Hundred Fists (Hands of Lightning) does 140% damage, + 28% damage every 2nd hit of 3

High-Damage Spirit Spenders:

For these, I would refer you to the question on What monk skill deals the most damage per spirit?, because you will want to get the most damage out of your spirit buildup. In general, Lashing Tail Kick (one of the first skills you get!) is an extremely effective and efficiency spirit-spending move.

All of the above can be used as a reference, but the true highest DPS build will depend on your gear and the boss monster you are fighting. Some of these skills affect areas or cones, and some involve movement, so the best skill will vary. Enjoy trying them all out!

As a primary attack: Way of the Hundred Fists, the forth skill.

It deals 140% of the weapon damage.