It's hard to gauge Luke. Vader didn't think Luke was skilled enough to be considered a Jedi and yet Luke held out against Vader's final offensive until Luke lost his hand. I'd say Luke did better against Vader than Kanan and Ezra did.
Ahsoka survived an encounter with Grievous, did well against Pre-Vizsla, but got destroyed by Barriss. I would put her beneath Ventress, who in turn is beneath TPM Maul.
I would also put Obi-Wan beneath Ventress and Maul though.
I would say Grievous is the closest thing to a Maul level opponent that Ahsoka had. She mostly fled from Grievous and she didn't really hold her own. Kenobi's offensive against Maul was more impressive than Ahsoka's defensive against Grievous.
So it boils down to either Luke or Obi-Wan. I'll go with Obi-Wan because of his formal Jedi training.