Goku: GT (SSJJ)

Great Ape
Like all Saiyans, Goku can transform into a Great Ape (Oozaru) when he sees the full moon while he possesses his tail, though every time it was not at will. Because this tail was removed permanently in the main series by Kami when restoring the moon, this transformation was never taken by Goku again until GT (in which it is shown very briefly before the full transition into Golden Great Ape), making it his least used Saiyan transformation.

Super Saiyan
Goku uses his new power to defeat Frieza, the antagonist of the series at the time. Frieza had destroyed the Saiyans out of his fear of their growing power, and the idea of a possible "Super Saiyan". As a result of defeating Frieza, Goku avenges the Saiyan race, and the death of his best friend Krillin, but it is Trunks who kills the tyrant for good in the main timeline (Goku did the honors in Trunks' own time, in which the time traveler obviously never appeared to warn the Z fighters of their impending doom).

Ascended and Ultra Super Saiyan
After having witnessed Vegeta and Trunks emerging from the Hyperbolic Time Chamber (Room of Spirit and Time), Goku concludes that surpassing the normal Super Saiyan form may be more possible than initially suspected, and enters the room to train himself and Gohan. After teaching Gohan to become a Super Saiyan, Goku is able to achieve this form alongside the further extreme of the form known as Ultra Super Saiyan. However, although this new powerup would not take as much of a heavy hit on his speed as the successor would, he decides that staying transformed like this demands far too much ki to be spent, and so decides that the regular Super Saiyan form is the best suited for combat, and makes a plan for him and Gohan to master it before they leave the chamber. As a result, after under 20 hours spent in the room (Vegeta and Trunks had spent a whole day each, and would spend another prior to the Cell Games), he and Gohan emerge as Full Power Super Saiyans. As a result of this and the new discovery of the most powerful Super Saiyan state at the time, Super Saiyan 2, this form is never used by Goku again, and Vegeta and Trunks soon follow.

Like the Ascended Super Saiyan form, Goku achieves this form in the Hyperbolic Time Chamber while training with Gohan. Although apparently stronger than Cell in this state, Goku told Gohan he could not defeat him because Cell was much faster, because "all the power in the world would do no good if you can't even hit the guy." Instead of persevering harder to simply increase power output, Goku decides that simply mastering the Super Saiyan form to suppress its downsides and make it a natural state is the best way to fight and gain power. This form is never used after its initial unlocking along with Ascended Super Saiyan, as a result.

Full-Powered Super Saiyan
After realizing the weaknesses inherent to the higher grades of Super Saiyan, Goku concluded that mastery over the basic Super Saiyan form would be more advantageous and went on to improve his endurance in that form, thus achieving this state. Though its never named in the main series.

Super Saiyan 2
When Goku transforms into a Super Saiyan 2 he has some noticeable changes, such as his hair growing longer and becoming even more spikier, making his hair more defined than his other previous Super Saiyan forms, along with disappearing hair strands surrounding the forehead which are exclusive to this form, like Gohan (but in a different way; for reference, see the picture). He also gains electricity in his aura surrounding his body, and his aura grows markedly larger and more vibrant in both the anime and manga.

Super Saiyan 3
Goku achieved this level in Other World as he could put unlimited energy into his transformation. Goku is first seen using this transformation against Majin Buu during the Majin Buu Saga. Once in this form, Goku was able to easily overpower Majin Buu in his first form and stood up to Kid Buu for a fair amount of time. However, the transformation is extremely draining in terms of energy, as it cut Goku's time left on Earth from hours to a stated thirty minutes after performing the transformation for a short time. When attempting to gather enough ki to defeat Kid Buu during their final battle, he was unable to sustain it as he had not taken into account the strain overcoming his living body, and soon reverted back to his base state. Altogether, these heavy disadvantages make it impractical to use the transformation out of the afterlife.

Golden Great Ape
Goku achieved this form in Dragon Ball GT, after Old Kai helped Goku to grow back his tail which allowed him to once again transform into a Great Ape. When he looked at the Earth during the Baby Saga, its sufficient substitute Blutz waves allowed him to take this form, a combination of the Super Saiyan state and the classic Great Ape transformation. Upon regaining control, he later transformed into Super Saiyan 4.

Super Saiyan 4
This form is achieved by Goku after achieving the Golden Great Ape state and being calmed down by Pan. The overwhelming power of this transformation is apparently great enough to overcome the Black Star Dragon Balls' power making Goku's transformation into a child by Red Shenron. The spell is only dispersed during the transformation, however. In addition to him being turned back into an adult temporarily, Goku's overall appearance changes yet again. His tail is easily visible, the hair is longer than when in his normal state (not nearly as long as a Super Saiyan 3, though), his eyes become lined (much like Vegeta's did when he was a Majin in the Majin Buu Saga), his eye color is now golden yellow, and red fur covers almost all of his body from head to toe. His clothes change from his childhood garb, leaving him only dressed below the waist (excluding his armbands).

Kid (GT)
Goku was accidentally turned into a Kid by adult Pilaf after Pilaf over-exaggerated on his wish intent making himself, and Shu their proper age again.

Super Saiyan (Kid)
Goku retains the ability to transform into a Super Saiyan. He retains his abilities, and can maintain it longer than Super Saiyan 3. Thanks to his tail being restored during The Pack Group; he is stronger than normally.

Super Saiyan 2 (Kid)
Goku used this form in order to end his fight with General Rilldo, and save his friends from the Machine Mutants.

Super Saiyan 3 (Kid)
When Goku attempts to battle Baby Vegeta after taking control of Super Saiyan God Super Saiyan Vegeta, but Baby Vegeta severely outclassed Goku especially after Baby Vegeta transformed into his Strongest Form 1. Due to his Kid size he was unable to maintain this form for long.