Son Gohan Sr. -- Gohan taught Goku the very barest basics in fighting, but Goku's fond memories of his old grandpa and sentimental love for the Four-Star Dragon Ball were a major driving force behind his earliest adventures, culminating in a tearful reunion at Baba's Palace. Although he died before the story even began, Gohan taught Goku some of his lifelong values, including to be nice to women. I'd say he's a strong contender because of how much influence he still has over Goku's life, especially in the earliest few arcs.
Muten Roshi -- I think Roshi has perhaps the best claim to the title, for being the first major one who got Goku into martial arts, instilling in him most of his values and goals in life, while also acting as a surrogate grandfather figure. The Kamesenryu fighting style and philosophies have stuck with Goku throughout the saga -- to move well, learn well, play well, eat well and rest well. On top of that, he schooled Goku and Kuririn in basic life skills such as mathematics, literacy and manual labour. Roshi set the standard for all of Goku's other teachers, continuing to dispense wisdom even in Dragon Ball Super. What's also notable is that Roshi was lax in teaching Goku how to fight, instead putting him through a gruelling training regimen to get him up to a superhuman level of strength, speed and agility, and allowing the fighting instinct to come naturally.
Karin-sama -- You can't forget the squinty cat in the tower! Renowned as a God of Fighting, Karin was Roshi's own mentor decades before the story began. He promised to give Goku a vial of Divine Water that would boost his strength, but only if he could take it from him. Easier said than done. The whole test was a placebo trick, as the Divine Water was just regular tap water. Goku gaining the agility to catch Karin was the real power-up all along... well, until the Piccolo Daimao arc, where Karin casually hands over the actual Divine Water for a cheap power boost. Kinda broke his own lesson there. Karin is still a pretty big deal, though he's mainly a Senzu-dispenser these days.
Kami-sama and Mr. Popo -- Much of Goku's training at Kami's Temple is a mystery. The time between Goku's first meeting with Popo and the 23rd Tenkaichi Budokai is almost entirely skipped over in the manga. Although he was also quite hands-off, Kami gave Goku one of his most valuable training tools: weighted clothing, an evolution of the heavy turtle shells Goku and Kuririn wore while training under Roshi. On top of that, Kami has the Room of Spirit & Time tucked away, as well as other cool training simulations in anime filler. After defeating Piccolo and getting a little arrogant, Goku was cut down to size by Mr. Popo, who ate his goddamn Kamehameha. Many fans speculate that Kami's teachings gave Goku a greater sanctity for life, though this is unconfirmed. Whatever the case, his time on the floating island marked Goku's overdue growth into a young adult.
Kaio-sama -- Another strong contender, Kaio is a rung above Kami on the divine ladder, being the god of the entire northern quadrant of the universe. A great matial artist and powerful telepath, Kaio considers himself more of a comedian, but to his chagrin, no one wants to hear his terrible puns. Kaio is a hands-on mentor as he directly teaches Goku two extremely potent techniques: the Kaioken and the Genki Dama. He also seems to give Goku limited telepathy of his own. Since Kaio's planet has much higher gravity than Earth, simply being there strengthens Goku greatly, so he still comes back to visit every now and then. Despite how much Goku owes him, Kaio tends to get treated like crap ever since his little planet became an unofficial bomb disposal site. While he never leaves his planet, Kaio is arguably the most proactive master, as his telepathy allows Goku and Vegeta to request energy from the people of Earth for the giant Genki Dama against Majin Buu.
The Yardrats -- A race of mystical aliens from Planet Yardrat who go completely unseen in the original manga. After helping him recuperate from his epic battle with Freeza, they give Goku one of his most useful techniques: Teleportation, which alone took him nearly a year to master. They make full appearances in the Super manga, where they play a larger role in teaching Vegeta about the art of Spirit Control.
Whis -- The God of Destruction Beerus's camp, unassuming attendant; actually his teacher and the strongest being in Universe 7. Despite being a recent character in the revival era, Whis is already Goku's most prolific master, mentoring him and Vegeta in the usage of godly ki from 2014 to the present day. From day one, Whis has been trying to drill the principles of Ultra Instinct into the Saiyans' monkey brains, acclimatising them to the concept of letting their bodies instinctively react to any danger. He also tries to get them to work together, but that lesson mostly falls on deaf ears. Thanks to Whis's teachings, Goku has learned several transformations, including Super Saiyan Blue, Ultra Instinct Sign and finally Perfected Ultra Instinct. Like Kaio, Whis is also very dependable in helping Goku whenever he's in trouble, rewinding time to prevent Freeza's destruction of Earth, saving Gowasu from Zamasu, and blocking a punch from Moro.
Merus -- A super-competent agent of the Galactic Patrol; secretly Whis's angelic brother disguised as a mortal. Merus also helps Goku master Ultra Instinct in his own Time Chamber entrance. Although Angels are only allowed to use a limited amount of their power in training contexts, Merus feels a moral obligation to step in against Moro's galactic rampage, making him one of the few mentor figures to fight an enemy head-on alongside the Z-Warriors. The risk cost him his life, but his noble sacrifice gave Goku the Will to Power...-up against Moro.
Daishinkan / Grand Priest -- Yeah, Goku training under Whis's dad is a development only seen in Super Dragon Ball Heroes, but I guess it deserves an honourable mention. In Heroes, Goku somehow escapes the exploding Prison Planet and briefly trains with the Grand Priest in mastering Ultra Instinct. Thanks to Heroes' breakneck pacing, we naturally don't see much of it, but it results in Goku being able to utilise Ultra Instinct Sign at will for the first time. Grand Priest also gives Goku some pretty good drip.
Goku himself -- Yep, this is peak meta. In the Namek arc, Goku wears a dogi with his own 'Go' kanji on it, symbolising his enlightenment after his high gravity training in Dr. Brief's spaceship. In other words, he becomes his own master. But even before that, he spent a long sojourn training himself in the wilderness prior to the 22nd Budokai. Although he loses the kanji, he continues to train himself throughout the Z era and beyond.
