That's definitely a tough choice once you consider everything.
Jiraiya taught him summoning, how to pull out Kurama's chakra and the Rasengan.
Kakashi taught his how to use shadow clones, the cheat method to spped up training and helped him develop his strongest move.
Fukusaku taught him Frog Kata and SM, which allowed him to throw FRS.
Bee/Hachibi taught him how to overcome hatred, how to take more of the 9 tails chakra than the one that adds to his reserves, and make it his own. Bee taught his the mechnics/ratio split for the TBB.
Each one built upon the previous one, so it's a taugh choice but I go with Kakashi.
The reason is because in all that time Jiraiya didn't teach him how the SCJ works, how he could take advantage of it, how to use his natural affinty or try and havr jim add it to the rasengan.