The worst ones are the ones you never saw coming, but should have.
The Jedi Council knew all about Qui-Gon Jinn. He was one of the best Jedi ever, but he was also fiercely independent, and that bothered the Council a great deal. They spent an inordinate amount of time scrutinizing Qui-Gon. The headstrong Obi-Wan Kenobi was assigned to him as padawan learner because although they seemed to have so much in common, Obi-Wan was much quicker to assume the Council knew better in its decisions, no matter how much he respected Qui-Gon.
That was as much as anyone knew or cared about what Obi-Wan was up to. Except for his relationship with Qui-Gon, Obi-Wan by definition was unimportant. He was just another padawan.
Except he wasn't. The Council, or at least Yoda and Mace Windu, had known for years that the Force was weakening. This could mean only one thing, the return of the Sith. They didn't know who he was, but they had a name, and that was Darth Sidious. Suspicion eventually fell on the ambitious Naboo politician Palpatine, who was noticeably interested in the career of Anakin Skywalker, who was, at least as Qui-Gon Jinn was concerned, the chosen one of prophecy who would bring balance to the Force. While it would certainly have made sense for Palpatine to be Darth Sidious, the horrible truth is that it was Obi-Wan Kenobi all along.
Like Palpatine, it was all about opportunity. Obi-Wan's was far more direct, and, well, insidious. He seemed to be just another recruit. When paired with Qui-Gon, though, he was immediately thrust into an opportunity to shape the future of one maverick. When Qui-Gon discovered Anakin, Obi-Wan suddenly had a chance to shape the future of another, a far more dangerous one. Qui-Gon almost immediately falls to Sith apprentice Darth Maul, Obi-Wan somehow defeats Maul, and he makes a successful argument to the Council that he obtain Anakin as his apprentice.
From a certain point of view, right?
Years later, Obi-Wan is struck down by Anakin, who has become Darth Vader. It is a defining moment in the fight of the Rebellion against the Empire. Anakin's son Luke is motivated to begin Jedi training as a result. Obi-Wan seems like a major failure for a great schemer, right? Except he's already manipulated everyone into the places he needs them to be. Why would he need to worry about no longer having direct influence? Anakin became a Sith. Palpatine has not only been assumed to be Sidious, but has risen to Emperor of the Old Republic, which he has totally transformed. Yoda is the last of the Jedi Council, and willingly went into exile. He has the ability to train Luke, who has long had a relationship with...Obi-Wan. Obi-Wan comes back as a Force shadow, completely accepted by Yoda as a peer. He is able to help shape the further development of Luke's new career. He helps push Luke into an emotional decision to face his father Anakin in combat well before he's ready. He knew all along Anakin's role as the chosen one could permanently end the Jedi or the Sith. It seemed the Jedi would finally be eliminated.
He did all he could.