I will say Minato. He cannot protect Obito from the boulder. And he cannot protect Rin from Anbu from Mist --- The Only Uchiha Izuna of the Storm 3 Board http://i46.tinypic.com/ngcxoz.jpg, http://img37.imageshack.us/img37/5568/gfaqs.png
Hiruzen wasn't able to stop Oro when he found out about his experiments and that was a big mistake. But Kakashi fumbled big time with Sasuke, as I believe that he favored him over Naruto and still didn't manage to kee him away from Oro. Also he never took Naruto serious and apart from praising Sakura before the chunnin exams, he also did little to nothing to forward her growth as a ninja.
Minato's failures got him and Kushina dead oh the irony. --- The death of one man is a tragedy, the death of millions is a statistic-Joseph Vissarionovich Stalin Official 5 Tails Jinchuriki Han of the UNS3 boards
I'm gonna say Gai here... Mainly because he mainly concentrates purely on Lee and it seems for most of the time, Neji and Tenten are pushed out. Neji has the natural Hyuuga ability and the power of the Byakugan plus his own resolve to become stronger and protect his friends and comrades which makes him strong. That is mostly nothing to do with Gai.
Tenten however hardly ever gets taught by Gai personally (from what has been shown) and appears to have a nearly non-existant friendship/relationship with her sensei, and because of that she's mainly useless. She's only good when she summons a million ninja tools from the scrolls but even then that doesn't do a lot to the enemy.
Gai concentrates purely on Lee, always trains with him and yes Lee is strong because of it, but a sensei should concentrate on all his students equally. And for that, I don't appreciate Gai as a sensei. --- XBL: DonkeyDood15 I'm stayin' alive, I spit in your eye, drive a stake in you.
Kurenai Was a newly appointed Jounin she was not ready to be a sensei.
She pretty much only focused On genjutsu her self making her a horrible choice for a teacher. Hinata Is a hyuga and does not use Genjutsu. Kiba Lacks the patience to Use it. And Shino Clearly only uses his clans abilities.
Not her fault really. It was More Sarutobi's For selecting her as a sensei.
Asuma Clearly trained his Genin Look at Ino And Choiji In part 2.
Kakashi Was a horrible sensei. He favoured sasuke and Still failed In Putting him On the right path. He Ignored Naruto and allowed him to get even dumber Crippling him as a ninja. He also Ignored Sakura making her useless In part 1. All that she Is In part 2 Is because of Tsunade and she clearly lacks the mental fortitude to be a front line fighter.
Minato was Ok But He was young and was one of the main trump cards In a war He just did not have enough time.
Hiruzen Is probaly the best sensei. Man Taught 3 stundents who all became legendary and powerful. Orochimaru was not his fault I mean he was around 40 when Hiruzen failed to stop him right? I dont think that at that age Hiruzen Is responsible for him.
Jiraiya Was a good sensei to minato but horrible with Naruto. 3 Years and all he learns Is how to stop Genjutsu? At least he trained the AME Orphans correctly.