Well if revan and the emperor were the only ones left is will be like there past 2 encounters all over again in the end revan eventually fails. Revan and vader would have a chance if vader didn't haft to lug around his personal bacta tank life support suit.
Isn't his tank sometimes a benefit in a fight? Say someone shoots blasts at him. If he didn't have the suit he would be easily gunned down if he missed deflecting a bolt. With the armor he is slower but it has added protection. Of Course he moves much slower. Which is why I think Vader would fight the Emperor because Yoda's speed would be too much.
Yoda while impressive has never actually beaten a high figure. He's came close and was impressive but he lost or was a draw all the fights he fought.