Everyone sleepin on Gai sensei?
All I'm seeing is, "C taught D, then D taught E, then E taught F, so C is the best sensei ever."
or, "Lets look at who C taught. C taught D, E, F, and G, who all ended up being super strong/kage..."
Lets disregard the chain teaching for a second here. A sensei can only be responsible for whoever he himself taught. What his student does after that shouldn't be counted towards that Sensei.
Lets also not give so much weight to the number of students a sensei has had because that would make Iruka the greatest Sensei in Konoha... Honestly, most of those students (and im referring to Jiraiya and Hiruzen specifically at this point) taught themselves. Granted Hiruzen instilled "The Will of Fire" into Jiraiya, and in turn Jiraiya instilled it into Minato and Naruto, that alone doesn't make a top notch shinobi. A sensei has to be able to make their student powerful, as well as put their student on the path of a shinobi.
All that being said, I don't think anyone is a better example of an excellent sensei that Maito Gai. Gai took Lee, who was hopeless in ninjutsu and genjutsu, and treated him as if he was his own son. Who can honestly say that when Naruto first came out, they didn't think Gai and Lee were related?
Gai took Lee and molded his abilities as a shinobi; making him one of the best taijutsu users in the series. Gai took Lee and instilled "The Will of Fire" in him as well. Gai taught Lee the importance of hard work. Gai taught Lee that never giving up is what makes a shinobi great. Gai taught Lee that friendship is more important than anything to a shinobi. Gai has made Lee one of the most righteous ninja in the entire series... He not only made Lee strong in body, but he made him strong in mind and character as well.
I would choose Miato Gai as my Sensei any day.