In Harry Potter and the Goblet of Fire, it was mentioned that Moody was defeated by both Bartemius Crouch Jr. and wormtail, but Moody is the most powerful auror the Ministry has ever had. How come? However, Dumbledore is much older than Moody is, yet Dumbledore can still take on Voldemort with no problem. Is that a plothole? Can someone explain?
As I explained in my answer to one of your other questions, Moody was not necessarily rusty. I noted that Moody was the victim of a surprise attack, it was two against one, and they still barely managed to subdue him. I also listed about a dozen other members of the Order of the Phoenix who lost a duel, or were overpowered in some way, over the course of the series. In fact, as I mentioned there, Moody was still considered the most powerful Order member after Dumblddore, as Tonks mentioned in the beginning of Deathly Hallows:
"You-Know-Who acted exactly as Mad-Eye expected him to," sniffed Tonks. "Mad-Eye said he'd expect the real Harry to be with the toughest, most skilled Aurors. He chased Mad-Eye first, and when Mundungus gave them away he switched to Kingsley...."
In short, Moody did not necessarily deteriorate more than Dumbledore. He just wasn't as powerful.