Dragon Ball Z: Kakarot's characters frequently gain dramatic spikes in power somewhat unexpectedly, typically whenever a new transformation is unlocked. Often, this ends up being the key to victory for the Z Warriors, as their foe tends to start out at a much higher power level than they do. That said, in most cases there is a clear definition of who the strongest character in a saga is by the end of it, and believe it or not, it isn't always Goku, in fact it rarely is.
Dragon Ball Z: Kakarot retells the story of Dragon Ball Z, and even keeps track of an approximate power level of characters at various points throughout the story. Unfortunately, during certain moments such as Goku's first Super Saiyan transformation or Vegito's battle with Super Buu, it is impossible to check the BP of a character. This is rather a representation based on their feats during each saga, and represents how drastically the top dog changes in the Dragon Ball franchise.
Saiyan Saga - Vegeta
Saiyan Saga Vegeta was far more powerful than any other character around at that time. While it is true that he ultimately lost to the Z Warriors, it was hardly a fair fight. Even with Goku's Kaioken ability being used to surpass Vegeta and win the infamous beam struggle they were engaged in, Vegeta still survived and came back with a yet more powerful transformation. Although Gohan turns into a Great Ape as well, his is nowhere near as powerful as Vegeta's who has trained to use the form. It seems clear that Vegeta outclasses the likes of Krillin, Yajirobe, Gohan, Piccolo, and his fellow Saiyans, only losing this battle due to the team work and desperation of the Z Warriors.
Frieza Saga - Goku
Goku's iconic transformation into a Super Saiyan will long be remembered as the most iconic moment in all of anime, and essentially defined an entire genre. There is no question that Goku is the most powerful character in this saga, even though Piccolo and Vegeta do get much more powerful through Zenkai boosts and Namekian Fusions. That said, there is one who is close in power to Super Saiyan Goku and could even be considered stronger under certain circumstances.
Frieza's Final Form (Full Power) is actually stronger than Super Saiyan Goku, but the form's limitations make it lacking in desirability. As the battle between Goku and Frieza rages on a burning planet, it becomes clear that Frieza is losing stamina as a result of using the Full Power form. Given that Frieza and Goku fought in a one on one fight (unlike his battle with Vegeta), it seems fair to say that the sustainability of Super Saiyan makes it more powerful than Final Form Frieza's Full Power since that form is so short lived.
Cell Saga - Gohan
Gohan surprises in this Saga by taking the number one spot over both his father and Vegeta. Gohan's climactic awakening of the Super Saiyan 2 form is due in large part to his friendship with Android 16 and the clever training strategies that Goku employed. The Super Saiyan 2 form can't be unlocked simply through training; instead the user must fully master the Super Saiyan form. When Goku and Gohan stay in Super Saiyan form for over a full year, they learn to control the otherwise unstable state and access what has been come to be known as Full Power Super Saiyan. This form is necessary to attain Super Saiyan 2.
That said, Super Perfect Cell is definitely in the running for the most powerful character of this arc. It is clear that he overpowers every other character in the saga given that he did even before his Zenkai boost, but is he stronger than Gohan? Although he ultimately lost to Gohan, Super Perfect Cell was distracted by the other Z Warriors when he met his demise, and Gohan had some sort of aid from his father with the Father-Son Kamehameha. Still, Gohan was clearly weakened and wounded and still managed to defeat the rejuvenated and powered up Cell.
Buu Saga - Super Buu (Buuhan) or Vegito
If fusion is considered, then the most powerful character in Dragon Ball Z: Kakarot's final story arc is clearly Vegito, who proves himself to be much stronger than even Super Saiyan 3 Gotenks despite only going Super Saiyan 1. This is because Goku and Vegeta's base forms greatly outclass that of Goten and Trunks' base forms. It is clear that Vegito was far more powerful than any of Buu's forms, especially when considering he likely was capable of going Super Saiyan 2 and possibly even Super Saiyan 3. It's a shame that Vegito may never be fully playable in Dragon Ball Z: Kakarot.
Ignoring fusion, however, Gohan yet again sits at the top thanks to his Ultimate Form which the Elder Kai helps him unlock. When Super Saiyan 3 Goku battles with Buu, he struggles greatly, believing the fight to be unwinnable, yet Ultimate Gohan's initial encounter with Super Buu after he had absorbed many other fighters puts him at the top. Still, though, Super Buu eventually absorbs both Gotenks and Ultimate Gohan, making him far more powerful than either of those two. This may constitute a fusion, however, as his power was obtained as a result of using others' power. In fact, Vegito even comments on this during the boss battle with Super Buu.
Strongest Character Moving Forward
The power scale of Dragon Ball Z: Kakarot's characters is constantly in flux, and that isn't something which will change in the DLC. Although the addition of Super Saiyan God will certainly level the playing field between Vegeta and Goku, a yet more powerful foe will appear, Beerus. Dragon Ball Super is essentially a tale of Goku and Vegeta trying to close the gap between themselves and the destroyer god Beerus. Even then, though, it is said that Beerus' attendant, Whis, is even stronger than he is.
Dragon Ball Z: Kakarot is available on PC, PS4, and Xbox One.