In Dragon Ball Super, few forms are as powerful as Goku's Ultra Instinct and Vegeta's Ultra Ego. While Dragon Ball is famous for its transformations, the two latest are more than just powerful upgrades like Super Saiyan God, they're reflections of Goku and Vegeta's characters that show how much the two Saiyans have grown.
First premiering in the reboot film Dragon Ball Z: Battle of Gods, Super Saiyan God was supposed to be the pinnacle of Saiyan power. In the film and in its subsequent manga and TV adaptations, the form is initially achieved through a ritual wherein five Saiyans channel their power into a sixth. The ritual was a solid addition to Dragon Ball's typical transformation formula. Instead of a new form representing Goku's hard work and dedication to training, Super Saiyan God represented the family Goku had built. There's a reason that the fifth Saiyan was Goku's as-of-yet unborn granddaughter, Pan. Unfortunately, though the ritual is an interesting addition, it's the form's only interesting aspect as it is mostly just a flat power increase. Even the ritual part of it apparently isn't necessary as Vegeta unlocked the form while training. Though it may have taken some time, Dragon Ball Super did eventually introduce forms that are not only more powerful than Super Saiyan God, but they're also more interesting.
Over the course of Dragon Ball Super, Goku and Vegeta train extensively with the God of Destruction Beerus and his Angel Whis. The culmination of their training are two powerful new forms that are perfect for the two fighters. From Whis, Goku learns Ultra Instinct. In addition to a blanket power increase, the perfected form allows Goku to anticipate his enemy's moves without thought. On the flip side is Vegeta's most powerful form, Ultra Ego which he learns from Beerus. With Ultra Ego Vegeta can absorb attacks and turn them into fuel, making him more and more deadly as his lust for battle increases. Far from just power increases, what makes the two ultra forms so special is that they are far more personalized.
One of Dragon Ball's biggest mistakes was introducing the concept of power levels. Instead of new forms granting new abilities, all they had to do was increase a character's power level. While this worked fine for Goku's first Super Saiyan transformation, it had diminishing returns. Ultra Instinct and Ego are great forms because they are so much more than just power level increases. Each form comes with unique abilities that perfectly suit its respective fighter. What's more, Goku and Vegeta can't copy the other's form. Goku just doesn't the ego for Ultra Ego and Vegeta doesn't have the raw instinct for Ultra Instinct. This is especially meaningful for Vegeta who has spent so much of his time feeling like he's living in Goku's shadow. Ultra Ego finally gives Vegeta the edge over Goku he's been searching for.
While none of this should suggest that previous forms like Super Saiyan God are necessarily bad, they just fall flat in comparison to the complete package offered by Ultra Instinct and Ultra Ego. Goku and Vegeta are such well-loved characters for a reason, and Dragon Ball Super's greatest decision was giving them two forms that played to their respective strengths.