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?