Goku was trained by the best teachers that we've seen but there's only one that seems to be deserved be called Goku's best master. Who do you think it should be?
Probably Popo or Kaio. Roshi and Whis's training were similar and iirc, their training involved doing chores work. I don't remember much of Popo's training besides learning to moving faster than lighting and the weights.
Imma go with Kaio. A lot of the stuff Goku learned from the others he doesn't really carry forward through the series, save for the KHH. But he still frequently trains on Kaio's planet, uses the Spirit Bomb regularly and pulls out the Kaioken when necessary. So it seems to me that Kaio's training had a more lasting effect.
-Gramps Gohan: Taught him the basics.
-Roshi: Turned him into a competitive fighter.
-Korin: Internal organs' conditioning.
-Popo/Kami: Chi control.
-King Kai: Utilization of his Saiyan potential (same gravity as planet Vegeta).
-Whis: Turned him into a God.
Kaio since the techniques he learnt from him had longer staying power than any of the others (bar the Kamehameha), plus the meaning behind it (Proper Ki control) would become a major part of Galu's training in the Rosat later on. Kaio also had the most relevance to the plot, constantly scouting out info and locations for the Zet-Senshi.
Whis' training of the so-called Ultra-Instinct is a rip-off of Mr. Popo's training, but then Mr. Popo did have a power-level of over 1,000 and didn't try to help against Raditz so he's kind-of a lousy teacher. 2 2 2
I'd wager that Kami's was most beneficial since it advanced Goku's understanding and ability to wield Ki and Ki is used to augment your body and take it to levels that aren't even possible relying primarily on muscle. All of the fundamentals Goku learned from it was probably the most beneficial. All King Kai did was power him up through training.