Vikings isn't a series with a clear main character - or even with a couple of them! Much like other true ensemble series like Game of Thrones, Vikings is a series that likes to kill off major players and shift focus throughout, keeping viewers unsure of who, exactly, they are supposed to be rooting for.
For most of the early seasons, it seems clear that Ragnar Lothbrok is the main character, but his wife and sons later seem to take center stage. And for most of the series, there isn't just one main character, but two or even three that vie for that protagonist position. Broken down season by season, though, it may be easier to see who is supposed to be the hero of the story.
Season One - Ragnar & Lagertha
The first season of Vikings sees Ragnar and Lagertha living as farmers with their two children, and Kattegat under the control of Earl Haraldson. The big question of the season is whether the Earl will keep sending his people to raid East, where the land is known but the pickings are slim, or allow Ragnar to take his men to the West. It's a risky proposition, but one that has the potential for huge rewards. As the season continues, Ragnar is able to make his voyage, discover England, and take over Kattegat, along with his brother.
In this season, Ragnar and Lagertha are the core characters, and there isn't much competition for the position, either. Rollo, Siggy, and Floki play some of the other biggest roles among the Viking characters, but the bulk of the narrative goes to Ragnar. Lagertha shares his space as the main character, though, as she often gets scenes of her own, and her defense of the homestead when Ragnar is away the first time is also a big part of the show - as is her ruling over Kattegat when Ragnar is gone.
Season Two - Ragnar & Athelstan
In the second season, Ragnar shares the focus with Athelstan, with scenes split between Norway and England. The season sees the Viking politics become increasingly more complex, as Rollo and Ragnar face off alongside Jarl Borg and King Horik. Ragnar's own situation changes, too, as Lagertha is replaced by Aslaug and his family grows, and the Vikings clash with the forces of Wessex in England, leaving Athelstan behind to forge relationships with the King and his daughter-in-law.
While there is a lot going on this season, Ragnar and Athelstan the clear center of it all. Ragnar is fighting his own battles, but the season tends to split the story - focusing about half of the time on him, and half on the Viking-Monk. Lagertha, after leaving, isn't seen for several episodes of the season (although her story is later revealed), and Bjorn is with her. Aslaug takes on a major role, but not enough of one to be considered the main character alongside Ragnar, and similarly, Rollo, Ecbert, and Floki may have larger parts to play, but those parts simply aren't large enough to compete.
Season Three - Ragnar
In the third season, Ragnar remains the focus, although this will be the final season that he is truly the main character. This season is split into two major campaigns - the first in Wessex, against King Ecbert, and the second in Frankia, as the Vikings lay siege to Paris. This is also the season where Athelstan returns from Wessex, and is killed by Floki.
Ragnar is, obviously, front and center for both of these campaigns, and all the usual main characters have large parts to play. But unlike in other seasons, there isn't any one character who truly takes as much of the focus as Ragnar himself. This may be in part because the main character list is so long, at this point, that there just isn't enough time for a split focus.
Season Four - Rollo, Bjorn & Lagertha
Although Ragnar is still alive in season 4, he no longer feels like the central character. This is in part because he is missing for the earliest episodes, having disappeared for around a decade (and there is a time jump in the show to cover this). When he does reappear, it is as an old man, who no longer has the kind of power and command that he used to - and of course, this is the season where he is killed in England.
Instead, the main characters are split. Lagertha takes on a lot of the focus, and takes Kattegat for herself, killing Aslaug along the way. Meanwhile, a surprising amount of focus stays in Frankia, with Rollo, as he learns how to work his way into power in Frankia, and adapt to the society here. This, for once, puts him as much of a main character as anyone else. Finally, Bjorn begins to step into the main role here, with the scene where he kills a bear with his bare hands, all the way through to his leadership of the army that takes revenge for the death of his father.
Season Five - Bjorn & Lagertha
By season five, Vikings has seemingly forgotten about Floki and Rollo, and Ragnar and Athelstan are dead, leaving only Lagertha as a main character still standing, and bringing Bjorn to the forefront of the series. Lagertha keeps the focus as ruler of Kattegat (battling King Harald and later Ivar for that position), with her relationships with Torvi and Astrid front and center. However, Bjorn is also a main character in this season, as are his explorations, his battles for Kattegat alongside Lagertha, and his time in Wessex with Alfred.
His brothers come close to being main character here, too, especially Ivar, who sets himself up against Bjorn and Lagertha and takes Kattegat for himself. However, Ivar seems to be more the villain than the protagonist, at this point, and Hvitserk is his sidekick. Ubbe, meanwhile, is a close contender, but hasn't quite come into his own, at this point.
Season Six - Bjorn, Ivar, & Ubbe
By the final season of Vikings, the focus is almost entirely on the Sons of Ragnar, and the war between them. However, all three of Bjorn, Ivar, and Ubbe manage to act as main characters this season, even as multiple other major character arcs are wrapped up. Ubbe has his own journey to Iceland and then to North America (where he discovers that Floki lives!), Bjorn is attempting to becoming King of Norway and rouse all the Vikings to protect themselves against the Rus, and Ivar is with the Rus, and his own intrigue there.
All three storylines are given roughly equal weight, with each of these sons of Ragnar at the forefront. And Ivar, unlike in the past season, becomes less of a villain and more of a protagonist - and even has some heroic moments as he battles to save Igor from his uncle Oleg. By his death, it doesn't feel as though he is the villain and Bjorn the hero, but that he, Bjorn, and Ubbe are all heroes of their own stories.