Darth Vader vs Ben Kenobi (ANH)

Did he? Never heard of that before, I heard he became very good at it but mastered? Even so, he still had 20 years of not using it very much and to keep a style like that up... You will probably need some experience.

Nope. As of ROTS, Obi-Wan is a Soresu Master, and one of the best. And by ANH, his Force Mastery is probably near or on par with Mace Windu's. I mean, all he had to do for nineteen years was make sure Luke didn't run in front of a bantha. That leaves plenty of time for practice.

Well...I don't think that 20 years without fighting would do anything to your lightsaber skill.

We've seen some people in the movies who were damn good at lightsaber combat without having that much fighting experience. Sidious didn't probably have a fight from TPM to ROTS times dispite that he could still kill 3 Jedi Masters. Dooku didn't have much practice between TPM and AOTC either and he still could play with Anakin and Obi-Wan and survive Yoda.

Now Vader (from Mustafar on):

a) he lost 60 % of his raw power according to Lucas
b) he lost both legs on Mustafar (so he won't be too fast)

And now imagine Ben as something like Dooku or at least ROTS Obi-Wan with 20 years more experience with the force. He would kill Vader.

Exactly.

Ben would win.

Ben would win but it would be a boring fight they fight slow and unskilled so i think that ben would win but it would just be boring to watch

Imagine Kenobi at Dooku's age. . . a total badass. . .