Ragnar Lothbrok is by most fans considered the main character of Vikings, even though he left the show in season 4. He enjoys a legend-like status among his people as he was the first who set his eyes on the western horizon and thus brought abundance and wealth to Kattegat.
Ragnar was a complicated man. He was a natural leader with a capacity to take his strengths and turn them against himself and his people in a heartbeat. The higher status he enjoyed and the more success he reaped, the deeper he sank into depression and despair.
Best: Ambitious
If it wasn't for Ragnar's ambition, the people of Kattegat might have never sailed out West and discovered all the various parts of the world, featured on the show. He risked his life when he stood up to the earl and publicly disagreed with his orders.
The living conditions in Kattegat were harsh and Ragnar wanted a better standard of living for his people. He eventually became the leader of the Vikings and was free to do whatever he wanted.
Worst: Impulsive
One of Ragnar's worst personality traits is his impulsivity and unpredictability. He murdered Yidu in a fit of anger, shocking his sons and even himself just moments after he had done the deed. He was the kind of man who didn't worry about what tomorrow might bring.
Even though Aslaug warned him against sleeping with her on the night they conceived Ivar, Ragnar acted on his impulses and did it anyway. He refused to deal with the consequences; if it was up to him, he'd leave baby Ivar to die.
Best: Adventurous
What lay behind Ragnar's ambition was in fact his adventurousness. His biggest desire in life was to travel and find a better land to farm.
He passed on his wanderlust to his children: Ivar traveled on the Silk Road, Björn made it to the Mediterranean, and Ubbe sailed across the entire ocean.
Worst: Unreliable
Because Ragnar was so focused on his grand plans of conquest, he failed to be a reliable husband and father. First, he deserted Lagertha and Björn for Aslaug, causing them to leave Kattegat and find a new home elsewhere. Unfortunately, he was an even worse husband to his second wife. He never really cared about her: he just used her to see the Seer's prophecy come true.
While he enjoyed being a father on some days, he was nowhere to be found on the next. He spent some time raising his oldest sons, especially Björn and Ubbe, but Ivar barely got any of his attention. In Ragnar's defense, though, all Viking men were what would today be considered an unreliable father.
Best: Open-Minded
Unlike Floki who was a zealous believer in the Norse gods, Ragnar was far more open-minded - not just when it came to religion, but also philosophy in general. He dared to question the status quo and had existential conversations with his enemies, such as King Ecbert.
Ragnar found a fellow non-judgemental friend in Athelstan. They openly talked about faith and life purpose without fear, which wasn't as common among Ragnar's people. After Athelstan died, Ragnar had every reason to be so depressed: nobody shared his open-mindedness the way the beloved Northumbrian monk did.
Worst: Self-Destructive
While Ragnar's aggression came in handy on the battlefield, he also turned his destructive tendencies towards himself. After Athelstan's death, he found himself to be completely hopeless, so he resorted to self-medicating with Yidu's opiates. As time went by, he was steadily becoming worse and worse. As all the power started weighing on his shoulders, he started losing control over himself.
His self-destructiveness reached new heights when he sailed to England for the last time, knowingly giving himself away into the hands of the enemy. Ragnar had a death wish and ultimately, it came true.
Best: Inventive
Ragnar shares one of his best traits with his son Ivar: he was an inventive man and consequently, an extraordinary military strategist. Not only was the Siege of Paris one of the most glorious battles because of the sheer power of the Viking forces, but it's also unforgettable because of Ragnar's ingenious plan to fake his death. It takes some serious creativity to think of that!
Additionally, Ragnar loved to use new inventions. In the pilot episode of Vikings, he showed Rollo a sunstone that later on helped him navigate the ships in his westward advances.
Worst: Hypocritical
Ragnar's words rarely matched his actions. He always talked really highly of his family and sons, but essentially neglected them and let others raise them. The same could be said about the Norsemen in general. He despised his brother Rollo for betraying his people, yet Ragnar did the same in the end.
When he sailed to England for the very last time with Ivar, he murdered his own people in his sleep, thus preventing them from entering Valhalla and going against everything he was supposedly fighting for his entire life.
Best: Fearless
No matter what situation Ragnar found himself in, he never let fear get the worst of him. He was a fearless warrior and didn't flinch even when he was on his way to meet his death.
In Viking's universe, being fearless was more than just a personality trait. It was a survival tactic. Vikings were notorious for not fearing anything, not even death, and that's what made them so formidable to the enemies.
Worst: Arrogant
Because he rose through the ranks within his community, Ragnar developed an arrogant streak and thought of himself as essentially untouchable. In season 2, he agreed to go raiding with Jarl Borg, only to ignore the oath at the very last minute.
When Borg retaliated, Ragnar was outraged. Because of his arrogance, he failed to see that Jarl's actions were merely a consequence of Ragnar's poor decision-making.