Jiraiya Sensei is my favorite,
I lved his character from the first series on through to his final fight with Pain.
The three reasons I chose him were
A.) Even though comical and pervy at times, he still posessed a keen sense of seriousness and strength,
B.) Out of the three Sannin level, he was the only one who was actually genuine (Tsunade cheats by concealing real identity.
Orochimaru is just evil.]
C.) He has trained some of the most badass ninja to date. (Minato,Pain..etc)
If it wasnt for jiraiya I would have chosen Orochimaru,Kakashi or Sasuke.