Thank you, Escape. I promise to confine the arrogance and insults. I open by saying a bold statement and will offer more if asked: Sidious is indeed the strongest of all time, confirmed in several sources, first in the original piece of EU Dark Empire. In that comic, Sidious manages to cheat death several times, forcing his spirit into other bodies. He devours the life force of six billion inhabitants of the world Byss but doesn't kill them, showing control. In addition, at the end, he squares off with Luke in combat and by this point, Luke is referred to as very powerful. Palpatine focuses his rage and annihilates a fleet of ships with a force storm.
There is never anything to indicate this is different from the Sidious we see in the trilogies.
Moreover, when he visits Korriban, he is hailed at by the Sith, mocked by several but in the end they acquiesce to his wish when he demands it of them, saying he gave up everything to the Dark Side...in another source, Dark Lord, I believe, it is revealed Sidious was prophesized as the greatest by the Ancient Sith themselves. The creator of the ancient Sith, Kevin Anderson, vile Dune destroying heretic he is, said he never intended Sidious to be below his creations.
Another argument is 'Why does he never show this in ROTS?"
Same reason why Yoda isn't leaping around Luke like a madman: Both technology constraints...and that he doesn't need to. We could have quiet control from sidious or he could milk a few giant cows, wave his arms and level some buildings...and with Yoda, vaporizing half of Coruscant would probably not be such a good idea.
I'll most certainly say Vader was weaker as was as a lot of Ancient Sith, but probably greater than a lot of others. At the boards I frequent, it's been a huge debate against him and Revan.