Marine Base Arc
Defeated Morgan
Buggy Arc
Clashed with Buggy, defeated no 1 or no 2 commander Cabaji
Usopp Arc
Defeated no 2 and no 3 commander, assisted in defeating no 1 commander
Baratie Arc
Lost to Mihawk 100 levels above main villain
Nami Arc
Defeated tons of fishmen crew, defeated no 2 commander, clashed with Arlong
…
Since the very beginning and up to this arc Zoro has been defeating at least 1 top enemy in every saga, in some even 2-4. In surprisingly many, when you look back, he has clashed with either the main arc villain or even someone stronger (Mihawk, Fujitora).
In that light
Skypiea
Braham
Wyper
Ohm
Enel
Wano
Sumo
Hawkins
Kamazou Killer
Kyoshiro Denjiro
Apoo
Queen
King
Big Mom
Kaido
Egghead
Kaku
Seraphim
Lucci
Kizaru or Gorosei