1. Zallow. Legends scaling is absurd compared to Canon. And Old Maul himself isn't even impressive by Canon standards.
2. In Canon, there's somewhat mixed evidence for if Kenobi grew or atrophied after RotS. Personally, I'd say Ben takes it in Canon, as we know Vader grew beyond Anakin in Canon, and Kenobi put up a solid fight against Vader.
3. Ventress. Holding off Mace, holding her own against Obi-Wan, and standing her ground against Anakkn all puts her above an adversary whose best feat is stalemating a pre-prime Kun.
4. Darth Kraft, dark lord of Mac n Cheese, should absolutely take this.
5. I'm inclined to say Dooku, but I also don't know all of Malgus' feats.
6. Grievous. Legends Vader is weaker and slower than Legends Anakin. Meanwhile, Legends Grievous was an absolute monster in a duel, matching the likes of Windu and easily defeating several (weary) Jedi on Hypori.