Sidious does throw his fight, but it wouldn't have been a stomping he gives to Mace Windu. I'd say a little of what happens, but people get uppity over spoilers. 🙂
I did a little bit of research on Darth Vader. I checked the official site (GL's mouthpiece on the internet) and found nothing other than Vader is considered 'formidable and powerful'. So I dug a big deeper and apparently the case is that Anakin had the potential to be the most powerful, but he didn't follow up on it. Now, after his loss to Obi-Wan Kenobi, his power has been crippled; Vader can now only use 20% of his potential power. So, yes, Vader/Anakin is indeed potentially the most powerful ever... but he isn't and never will be, thanks to his own emotional faults, lack of discipline, and his loss to Obi-Wan.
http://www.pointlesswasteoftime.com/film/swflash.html
Funny link. Warning, don't read it if you're sensitive and don't have a sense of humor.