Borbarad
Advocatus Diaboli
Hmm...interresting...
I think some people should not judge the OT characters by their appearance in the OT movies only.
Vader
We know that Anakin / Vader lost a great amount of his natural potential in ROTS. But he gained force knowledge (Dark Side) and control. And the last thing is important. In ROTS when he still had his entire potential he wasn't able to overpower Obi-Wan (seen when they try to force push each other in their duel on Mustafar) - the very same Obi-Wan that was nearly singled out for the agricultural corps because of his lack of force potential before becoming a Padawan. So here we have the upper limit in terms of potential (Anakin) stalemating the bottom limit (Obi-Wan) in a force contest because Obi-Wan had 20 year of training advantage (meaning control).
So even if he lost great parts of his potential he might have been able to compensate that by gaining force control. The Emperor and Vader trained a nice amount of Dark Siders in the 2 decades of their reign and still Vader remained the unquestioned number 2 of the Empire - which actually shows us that he is still more powerful than the other people (including trained PT Jedi like Jerec).
We also know that Vader lost some amounts of speed / dexterity but even here you have compensation. First he gained pretty much strength due to his cybernatic body parts. Then he had his armor which partially protected him against lightsaber strikes (otherwise Luke would have cut his arm off in ESB when hitting him).
And he did refine his lightsaber style. Of course he still had his form V mastery at hand. In RoDV you have Jedi Master Roan Shryne noticing that Vader started to introduce form III and IV movements into his style and his fight against Luke in ESB (partially one-handed fighting) can lead to the suggestion that he experimented with form II too. At the very least Vader can't be easily tooled in lightsaber fights. In RoDV and Purge you have him fighting multiple Jedi at once. In one situation he even gets his weapon arm cut off, picks his own seperated limb up and kills the Jedi who did cut if off with the weapon that remained in his hand. In Purge after Jedi Master Shadday Potkin disabled his lightsaber using a cortosis blade he simply picks her up and breaks her neck.
And apparently he handled his lightsaber precisely enought to give Kir Karnos just a scar on his face while cutting through his face mask (and he wanted exactly that).
If Vader really was as slow and weak in the force as some people seem to have in their mind some Jedi he hunted should have tooled him e.g. the Dark Women who was over a century old (and still in great physical condition) when Vader found her. Instead he threw a tree at her...
Obi-Wan
Obi-Wan did more then just sit on his ass for 20 years on Tatooine. In "The Last of the Jedi" he escapes Inquisitor Malorum as well as Boba Fett and is forced to eliminate Inquisitor Sancor. Obviously that gave him enough experience to learn "the way of the Whills" from Qui-Gon's spirit. Remember that the Whills (in SW mythology) are the "history keepers" of the SW galaxy, most likely "higher beings" who are suspected to be immortal. And Qui-Gon learned the secret how to remain as a spirit within the force from one of them (and some other stuff).
And if you read the "Jedi Prince" novels apparently Obi-Wan managed to enter the spice mines on Kessel in 7 BBY, kidnap Ken (Palpatine's grand child) and move him to the Lost City of the Jedi on Yavin 4. If Kenobi gained knowledge about the Lost City which was abandoned even before Sadow's time and just a legend even among the Jedi Order and managed to move into one of the Empire's protected industrial facilities and "steal" a child basically unnoticed I'm quite sure he gained some huge knowledge during his Exile.
Also remember that he spent his time in one of the most dangerous places in the entire Galaxy filled with Tusken, Jawas, Krayd Dragons and other inconvenient surprises.
So while Dooku is the same he was in ROTS, Anakin / Vader and Obi-Wan have gained more force control, more force knowledge and had enough opportunities to develope their lightsaber abilities. I really don't think that Dooku can tool them as easily as he did in AotC or ROTS. Either Dooku loses this or he will have a damn hard fight against those two.