There is always something about anime that makes it fun and super exciting, and that Is; transformation. When an anime character transforms there is always an increase in power level, it might be a great or just a little increase but fact remains that that power level would be used to overcome obstacles.
Here are top 10 anime transformation
10. All might's transformation (my hero academia) 
All might as well all know posses the quirk "one for all" but this ability of his allows him change form. His usual form is that of an extremely thin person but with his quirk he is able to transform himself to be full of muscles and with his transformation his power level is greatly increased and is virtually unstoppable.
9. Dark lord Satan's transformation (hataraku maosama) 
The transformation of dark lord happens when he has absorbed the fear of those around him, his transformation increases him physically and also greatly increases his power level. Though his transformation was his usual form, since he came to earth he was reduced to just a normal guy. But with inflicting fear he will definitely transform.
8. Eren's transformation to a titan (attack on titan) 
Eren's transformation is indeed a popular one his transformation involves him changing into a titan, he can command other titans and he transforms by inflicting pain to himself.
The first time he changed to a titan, he was eaten by one of them, which definitely cause him lot of pain.
7. Alucard transformation to a hellhound (Hellsing ultimate) 
Alucard; the immortal vampire possesses various transformation, amongst which is the hellhound. Alucard transformation to the hellhound is indeed a great one he can consume his enemies while healing himself.
If you go against him you'll know why he's the "monster whose power radiate with darkness that cast a shadow on darkness itself".
6. Naruto's transformation to the sage of six path (Naruto Shippuden) 
Naruto has gone through lot of training, one of which gave him the power of the sage of six path, and he was able to master it. Though there is no significant increase in size, there is definitely a change in form and that form grants him incredible power
5. Luffy's gear change (one piece) 
Luffy gear change is indeed an incredible one. Presently he has four stages of transformation, being a rubber man he combins his ability with haki, with this he is incredibly strong and cool looking.
4. Ichigo kurasaki's transformation the final getsuga tensho (bleach) 
Ichigo kurasaki's transformation was brought about by him unlocking the final stage of his zampakuto. He converted his immense spiritual pressure into physical powers.
3. Escanor's transformation (seven deadly sins) 
Escanor the deadly sin of pride can transform himself with the power of the sun, with this power he remains the strongest member of the seven deadly sins.
2. Gon's transformation/ growth spurt (Hunter x Hunter) 
Gon's growth spurt was brought about by pure anger and the want to destroy pitou, the character that killed his friend by first of all cutting his hand then his head.
1. Goku's transformation into ultra instinct (dragon Ball super) 
Goku transformation is by far the most strongest and reliable transfomation, yes there is no significant change is form or size but it's effect is clear. To be able to use a move that even gods find difficult. The change will just be the hair and the aura around his body.