Well, if we're talking about general combat ability, then I'm not sure, if Saesee Tiin is not underrated a bit.
he sparred equally with Mace (yeah, it's just sparing, I know). He was probablyone of the best (if not the best) telepath in the order. And his TK skill were close to Mace's. But his raw power in TK was greater then Mace's.
So, I think, that in the force he's above Kenobi, Bulq, Kolar. Maybe he's even better then Plo. His only fail, is his fight with Sidious.
Anyway, it looks like Opress is on top Jedi Master's level, since he's maybe just a bit better then Ventress, a bit better or equal to Plo, and clearly weaker then Maul or Kenobi overall.
So for me, its like that:
Tier 1: Yoda
Tier 2: Mace, Anakin
Tier 3: Kenobi, Maul and possibly Bulq
Tier 4: Opress, Koon, Saesee, Fisto, Ventress
Tier 5: Drallig, Ti, Bondara, Mundi, Koth, Billaba (maybe tier4)
Tier 6: Unduli, Gallia,