Overall? Yoda, and I don't see how that's even debatable.
Under Yoda's direct watch, one of the most powerful Sith Masters of all time rose to power and affected the slaughter of thousands of Jedi -- nearly driving the entire Order("at the height of their power"
into extinction... And if fallen apprentices are a measure of a Jedi's failure, as Luke seemed to think, then Yoda was also responsible for the training of Count Dooku, whose Separatist army murdered untold billionS.
...Luke lost a dozen students in all(some were killed; some turned to the dark side) -- a drop of water in an ocean compared to Yoda's failure.