I really dont know about who is the strongest, but IMO i feel that Gai is far more supperior than Kakashi. I mean he taught Rock 3 forbiden techniques the gates, secondary lotus, and primary lotus. If Gai was able to teach rock these kick azz styles then i would guess he himself has perfected them. Rock opened 5 of 8 gates at a Genin lv and he was hella powerful, imagin if Gai had done the same at a Jounin lv. True Kakashi is Da Bomb, and knows over 1k jutsu but i dont even think he would wanna copy Gai'z techinques.

hehe but hey im just mouthin off here. i could be wrong