Not to mention that Vader and Obi-Wan are almost identical as personal combatants on top of that.
What about Darth Caedus? His flagship, the Anakin Solo would make a good matchup for the Executor. Other elements from the Galactic Alliance Navy could make up the rest of Caedus' fleet (they're MC80 cruisers, Endurance fleet carriers, and Nebula-class Star Destroyers for the most part).
Throw in Garen Malek as Vader's apprentice and Ben Skywalker as Caedus' and this could get interesting.
Caedus also has the Galactic Alliance Guard, his personal secret police and intelligence network. Darth Vader has his probe droids, his skills as a fighter pilot, etc.
For the sake of clarity, perhaps Vader should be assessed at the arguable peak of his power, sometime around the time of Empire Strikes Back.
You'd have to throw Tahri into this as Caedus' apprentice, remember Ben Skywalker quit being loyal to Jacen at the same instant that Jacen killed Mara Jade Skywalker and became Caedus.
Ben was never really Caedus' apprentice, and only seemed that would happen in 1 book (Inferno) and if it wasn't for a complete fluke Ben actually would have ran the fledgling sith lord through the back with his lightsaber...
Plus you'd have to throw in the factors of Luke Skywalker and such, because Ben and Jacen were both grandchildren of Darth Vader...