Yeah, like NewGuy said, shortly after RotS. He's obviously more powerful in RotS that he ever was before, and it's stated that he "lets go" of everything in his fight with Anakin, making him much more powerful. Then, there's the stories immediately following RotS, which show him more powerful than he's ever been before. There's also sources stating that by ANH, he'd grown older, weaker, more rusty.