Hard to say, to be honest, I can just said Zoro won't ever be on the list if people are objective, he mostly use his strength, if we talk about skilled swordman, Mihawk from One Piece will be a better choice, but still from what we see, it's not much skill.
Not the best but here a list and not ranked in any way:
Guts (Berserk)
Griffith (Berserk)
Oetsu (Bleach)
Unohana (Bleach)
Claire (Claymore)
Théresse (Claymore)
Zi Yu (Feng Shen Ji)
Wang Ling (The Daily Life of The Immortal King)
Allen Rodol (The Failed Swordman)
Ascheriit (Übel Blatt)
Ikfes (Übel Blatt)
Kirin (666 Satan)
Cid Kageno (The Eminence in the Shadow)
Orsted (Mushoku Tensei)
Sakata Gintoki (Gintama)
Takeshi Yamamoto (Katekyoshi Hitman Reborn)
Yato (Noragami)
Kenshin (Rurouni Kenshin)
Yoda (Star Wars)
Sephiroth (Final Fantasy 7)
Yun Che (Against The Gods)
Su Ping (Astral Pet Store)
Lin Huang (Monster Paradise)
Gu Qing Shan (Worlds' Apocalypse Online)
Kraugel (Overgeared)
Shi Feng (Reincarnation Of The Strongest Sword God)
Runihoril Sylnorin (My Own Novel

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