Mihawk's Iceberg slash is overall greatest feat in the entire series as @Shuyaku said
-On point blank Range it can be scaled to Island level due to the fact that mere shock waves are Large Mountain Level
-The attack also has insane lethality
-Mihawk was extremely casual when he pulled off that feat.
There is no feat in One Piece so far that matches up to that. Closest thing is Prime Garp busting 9 mountains but again busting 9 mountains warms Garp up, Mihawk busts a large mountain without even attempting to bust a large mountain.