I'd rank some of my choices like this:

1. Kaido's Thunder Bagua/Raimei Hakke (It's the same attack, but better to clarify names);
2. Big Mom's Ikoku;
3. Garp's Fist of Love (We haven't seen his biggest feats, but his reputation implies this);
4. Roger's Kamusari (The only named attack we have seen from the Pirate King, but still);
5. Elisabello III's King Punch (Said to be able to knock down forts, and we saw its power);

I should mention that I'm considering only purely physical attacks, and not including something like Devil Fruits, since those would be nearly impossible to consider i.e. Kuma's ability to deflect anything or Borsalino's lightspeed kick, as well as Zunesha's trunk swing and Kaido's Boro Breath. Also, there are plenty of powerful characters who do not have a single KO move, but have a rather destructive arsenal, like Sakazuki and Whitebeard/Blackbeard.