Specifically, I was looking at single-target damage and not considering status effects. And the top-damage power looks like Rain of Blows, a level-3 encounter power.
The reason is that it gives you the most attacks against a single target of any fighter power, which means you get to apply the weapon, strength, and feat bonuses to damage the greatest number of times.
You do have to be using a light blade, spear, or flail for the full benefit, but I compared it with other powers assuming that they also were used with their preferred weapons. In order to compare powers of different levels, I just went with a level-30 character using them. I assumed a final strength of 26, a +6 magic weapon, and +1 damage from weapon focus -- all pretty basic stuff for a level-30 fighter.
Assuming the fighter hits about half the time, that means an average of three attacks from Rain of Blows (two initial attacks, one of which connects to allow a secondary attack).
Assuming the highest-damage weapon (2d6), each hit would be for 2d6 + 8 (str) + 6 (weapon) + 1 (feat) = 22 average damage, or 66 for 3 hits.
No Mercy, the level-29 daily, would hit for 2d6 * 7 + 8 + 6 + 1 = 64 average damage.
If you insist on the +3 proficiency bonus and only go with swords, the best options would be a rapier for Rain of Blows and a Bastard Sword (two-handed) for No Mercy. In that case, the damage for Rain of Blows is 1d8 + 15 = 19.5 * 3 = 58.5, and the damage for No Mercy is 1d10 * 7 + 15 + 1 (versatile) = 54.5.
Rain of Blows even comes out better on crits. While No Mercy gets a big bump from all those weapon damage dice that can be maximized, Rain of Blows can crit multiple times and thus get the +6d6 crit bonus from the magic weapon multiple times.
There is one power that can compete (that I found): the level-15 daily Dragon's Fangs. That gets you two attacks for 3[W] + strength instead of three attacks for 1[W] + strength. For that level-30 character, Dragon's Fangs comes out ahead if you're using a 1d10 or larger weapon. It also does half damage on a miss, which puts it well over the top, but there are other factors that muddy the situation. For example, rerolls or attack bonuses would disproportionately help out Rain of Blows, because anything that helps you land the initial attacks means that you'll get more secondary attacks. Damage bonuses (e.g. the +4 from Kensei) also help Rain of Blows more, since you get to apply them on every hit.
(Note that the "average damage" I'm listing is the average damage when you hit. For true average damage, all those numbers should be multiplied by your chance to hit, but that doesn't affect their relative values.)
The reason is that it gives you the most attacks against a single target of any fighter power, which means you get to apply the weapon, strength, and feat bonuses to damage the greatest number of times.
You do have to be using a light blade, spear, or flail for the full benefit, but I compared it with other powers assuming that they also were used with their preferred weapons. In order to compare powers of different levels, I just went with a level-30 character using them. I assumed a final strength of 26, a +6 magic weapon, and +1 damage from weapon focus -- all pretty basic stuff for a level-30 fighter.
Assuming the fighter hits about half the time, that means an average of three attacks from Rain of Blows (two initial attacks, one of which connects to allow a secondary attack).
Assuming the highest-damage weapon (2d6), each hit would be for 2d6 + 8 (str) + 6 (weapon) + 1 (feat) = 22 average damage, or 66 for 3 hits.
No Mercy, the level-29 daily, would hit for 2d6 * 7 + 8 + 6 + 1 = 64 average damage.
If you insist on the +3 proficiency bonus and only go with swords, the best options would be a rapier for Rain of Blows and a Bastard Sword (two-handed) for No Mercy. In that case, the damage for Rain of Blows is 1d8 + 15 = 19.5 * 3 = 58.5, and the damage for No Mercy is 1d10 * 7 + 15 + 1 (versatile) = 54.5.
Rain of Blows even comes out better on crits. While No Mercy gets a big bump from all those weapon damage dice that can be maximized, Rain of Blows can crit multiple times and thus get the +6d6 crit bonus from the magic weapon multiple times.
There is one power that can compete (that I found): the level-15 daily Dragon's Fangs. That gets you two attacks for 3[W] + strength instead of three attacks for 1[W] + strength. For that level-30 character, Dragon's Fangs comes out ahead if you're using a 1d10 or larger weapon. It also does half damage on a miss, which puts it well over the top, but there are other factors that muddy the situation. For example, rerolls or attack bonuses would disproportionately help out Rain of Blows, because anything that helps you land the initial attacks means that you'll get more secondary attacks. Damage bonuses (e.g. the +4 from Kensei) also help Rain of Blows more, since you get to apply them on every hit.
(Note that the "average damage" I'm listing is the average damage when you hit. For true average damage, all those numbers should be multiplied by your chance to hit, but that doesn't affect their relative values.)
