Every martial artist dreams of having the kinds of teachers found in Dragon Ball. Who wouldn't want to be able to do a Kamehameha? Goku and the Z Fighters were all able to learn from lots of masters over the years but few are on par with the venerable King Kai.
Anyone can train with King Kai as long as they are given the honor of keeping their body and can cross Snake Way. Oh, and if they can tell him a good enough joke. Those who have managed to complete both these tasks have massively powered up showing just how much it's worth making the trip to King Kai's tiny planet.
10 Yamcha
After a humiliating defeat on earth to a puny Saibaman, Yamcha really needed a chance to level up. Fortunately, he had time with King Kai after his death while waiting to be revived by the Dragon Balls. His training had to have been extremely successful, as when the Ginyu Force arrives on King Kai's planet to cause problems Yamcha is able to take down Recoome with relative ease.
9 Olibu
All of Earth's heroic legends are apparently based on Olibu and Korin Tower was even apparently built in his honor. This blonde adonis reminiscent of Hercules has had 10,000 years to train, some of them with King Kai, before Goku eventually meets him. He still isn't stronger than Goku despite all that time which shows just how mind-bogglingly strong the Saiyan warrior is.
8 Goku
Goku was the second person ever to run the entire length of Snake Way and reaching King Kai was absolutely worth it. While a majority of his training was really done by Bubbles and Gregory at first, the techniques he learned from the Kai help him through the majority of battles he faces throughout the series.
How many characters have been taken out or severely weakened by a well-placed Spirit Bomb? How many more have fallen due to Goku powering up immensely with the Kaio-Ken technique? There's a reason Goku still keeps King Kai's symbol on his gi.
7 Krillin
Unlike most of the other Z Fighters, Krillin doesn't have the chance to meet King Kai until far later in the series. After he is eaten by Majin Buu, Krillin takes the time to train with King Kai until Kid Buu appears. He risks his very existence trying to launch one of his signature Destructo Disks at him to no avail, but is still able to hold on until Goku and Vegeta can save him.
6 Sarta
Sarta died protecting his home planet from meteorites over three thousand six hundred years ago before he met Goku for the first time. He was given the honor of keeping his body and being allowed to train with King Kai long before the Saiyan visited his small planet. He doesn't manage to make waves during the Other World Tournament, but he still is an impressive warrior.
5 Chiaotzu
After his self-destruction failed to defeat Nappa in the battle against the Saiyans, Chiaotzu needed a chance to get his power level up. Kami's influence allowed him to retain his body upon death and travel Snake Way to train with King Kai. Just how far did he come from the master's training? Seeing as how he demolishes Guldo of the Ginyu Force with incredible ease, we'd say pretty far!
4 Piccolo
While Piccolo crossed Snake Way with many of the other Z Fighters, he was reluctant to train with King Kai. He was able to catch Bubbles almost immediately (when it took Goku forty days to do so) and it made him feel like the training would be useless afterward.
In the anime, he comes to regret this decision. Piccolo ends up having a sparring match with Tien, Chiaotzu, and Yamcha, he remarks on just how powerful they all have become under his tutelage. Sorry Piccolo, it's too late to go back now!
3 Mijorin
Furries' tails must have been wagging when Mijorin was introduced to the series. He fought off an evil army of invaders trying to hurt his home planet over 7,000 years ago and is thus given the honor to retain his body and train with King Kai. He's one of the few characters from the Other World Tournament arc to actually make an appearance in Dragon Ball GT though it shows him training with the South Kai. If only his original master knew...
2 Tien Shinhan
He is among the Z Fighters killed by the Saiyans after his Spirit Tri-Beam isn't quite powerful enough to take down Nappa. While he has a hard time with the joking portion of his training (seriously, his joke was terrible) he was able to power up beyond belief thanks to King Kai. He was able to defeat both Jeice and Burter at the same time which is no easy task.
1 King Yenma
Many people forget that King Yenma was actually the first person to ever run the entirety of Snake Way. Considering how easily he can dispatch Raditz, it makes sense that he once received training from King Kai. At the time, he is shown to be stronger than Goz and faster than Mez as well. His power level might not be as much later on in the series, but upon meeting Goku, his strength is beyond impressive.