We can almost unanimously agree that compared to the imagination of the lore masters and junkies the depiction of the thu'um in Skyrim is pretty lackluster.

There have been pretty extreme examples of thu'um since Morrowind. Wulfharth used "Shake the Dragon" to reverse time, but was killed in the process. Hjalti changed the landscape of Cyrodiil though it's common belief that he used CHIM to achieve that. Then there's Reman with the "Emperor Voice". In Skyrim Alduin uses "Slen Tiid Vo" which I suspect is a dov variant of "Shake the Dragon". In Solstheim the LDB learns "Bend Will" and "Dragon Aspect".

So in-game limitations aside; what is the most powerful shout?

"Shake the Dragon" didn't reverse time, it seems to have done the opposite and sped it up. Wulfharth used it to return the years Alduin had eaten to the Nords and he used it for too long and died from the shout making him extremely old.

I thought it reversed time to the point before Old Knocker and Alduin stole their years. My mistake.

bend will seems like a strong candidate for most powerful shout