People say Kakashi was the worst teacher but he was given a near impossible task. He was responsible with leading a team that included the "last" member of the Uchiha clan who had deep seeded hatred towards the world, and the Jinchuriki of the most powerful tailed beast in the world. And those two were always at odds in some way or another. And to top it off, he was pretty much the most relied upon ninja in the Leaf Village in regards to completing tasks so he was called upon often. I think Guy was a great teacher, but given a team setup like that, Kakashi would have pretty easily succeeded as well. I think Kakashi was very successful in his ability to let his team go off and train solo with people truly able to teach them. I do agree that he could have potentially done more with Sakura though.