Hardest single hit action? (non-dot)

[Kabooa]

It's Kaiden Stesugekka. Foul is about 10% weaker, buuuuuuut it's also AOE, and that's where it will dumpster Setsugekka.

[Zerathor]

It's Kaiden Stesugekka. Foul is about 10% weaker, buuuuuuut it's also AOE, and that's where it will dumpster Setsugekka.

Pretty much this. I think the highest foul in a raid I've ever seen with TA, contagion, ballance, foe, and pot was 50k dh crit. I believe I've seen a 65k sam hit

I'm wondering if that's not Foul (650 potency)

If we look at it with the raw potency itself, it's Midare Setsugekka indeed (720 potency).
However, to calculate the strongest hit, we have to consider passive traits and self buff.

Samurai has a 10% self damage buff to setup on himself and another 10% damage debuff on the target.
Black Mage has a passive trait that increase every action's damage by 30% and Enochian which increase damage by 20%.

Foul: 650 potency. Enochian multiplier 10%, Magick and Mend II 30% boost
650 * 1.1 * 1.3 = 929.5

Midare Setsugekka: 720 potency. Kaiten multiplier 50%, (I'm sure there's another multiplier, but I don't play sam and don't know it)
720 * 1.5 = 1080

Midare Setsugekka just wins that battle on single target.