Among all the characters in Vikings. Lagertha is arguably granted the best character development. Fans get to see her grow from a shieldmaiden to the Earl of Hedeby and finally the Queen of Kattegat. Lagertha also has some of the best relationship arcs as she dates several strong characters and has relationships with both men and women.
During her time in the series, Lagertha is romantically involved with six different characters. In her list of lovers, some are portrayed as quite adorable while others keep angering fans in all types of ways. But no matter how good or bad they are, all of Lagertha's lovers end up having a major impact on her life.
Earl Sigvard
After divorcing Ragnar, Lagertha gets married to the Earl of Hedeby. Sadly, the marriage turns out to be a terrible one. Sigvard is shown to be an abusive husband who terrorizes both Lagertha and Bjorn. Eventually, she stands up for herself and stabs him.
There is hardly a thing to admire about Sigvard. He is often drunk and values his guests more than his own family. Despite Lagertha being one of the best shield maidens in Vikings, Sigvard doesn't support her and is quick to berate her whenever she chooses to go to battle rather than engage in domestic duties. He isn't confident that he is a worthy lover either, hence he constantly accuses Lagertha of still being in love with Ragnar. Generally, there isn't a single moment where Lagertha and Sivard are shown to be a loving couple. When his demise happens, there is more glee than melancholy.
Kalf
Kalf usurps Lagertha while she is away in battle but when she comes back, he claims to love her. Despite being advised to banish her, he keeps her as his co-ruler. Everything appears to be going well but in a surprise twist, Lagertha stabs him on their wedding day.
Kalf is definitely one of the Vikings characters who didn't get fitting endings but he only has himself to blame. Taking power from Lagertha then expecting her to love him unconditionally doesn't make much sense. And by taking power from her while she is away, he proves that he only has his own interests at heart. She even warns him that she will kill him one day but he still insists on marrying her. Usurping Lagertha is also something Kalf didn't have to do as she once hints that she would have accepted his offer for marriage if he was still her advisor. In the end, greed makes him lose everything.
Bishop Heahmund
Lagertha and the Bishop of Sherborne begin seeing each other after the first battle for Kattegat. She stops Ubbe from executing the holy warrior and when they bond over philosophical conversations, they find themselves falling in love.
Judging from his history, it's clear that Bishop Heahmund is only with Lagertha to satisfy his sexual urges. He is shown to be a lustful man who doesn't mind sleeping with anyone, including vulnerable widows, who look up to him for guidance. That he isn't proud of his relationship with Lagertha also qualifies him as a poor lover. He is quick to murder Lord Cuthred when the consigliere to Aethelwulf discovers the affair. Moreover, he dreams into irredeemable territory when he unceremoniously dumps Lagertha after having a vision of hell.
Astrid
The shieldmaiden begins sleeping with Lagertha in the fourth season. She later marries Harald and becomes Queen of the Kingdom of Vestfold-Rogaland but opts to die rather than live a life without Lagertha.
Astrid is not the perfect lover but there is never a doubt that she loves Lagertha. When she starts seeing Bjorn, it's only because she is upset about Lagertha shutting her out of her plans. She can't stand the fact that she is being shut out of important plans. And among all of Lagertha's lovers, it is with Astrid that the ruler has her best sex life. Additionally, Astrid doesn't get married to Harald out of her own will. She is forced to do it against her own will and by choosing to die rather than live a life without Lagertha, she becomes a true Romeo and Juliet kind of martyr for love.
King Ecbert
Lagertha gets into a relationship with King Ecbert when she remains behind in Wessex to work on the lands the English ruler has given as part of his deal with Ragnar. He is shown to be a good partner but when he broaches the idea of her staying with him forever, she refuses.
Ecbert is a maestro of Machiavellian schemes but as Lagertha's lover, he hardly does her wrong. He showers her with gifts and is often flirtatious with her. He is totally honest with her about his feelings and she appears to value him too. His proposal turns out to be the only dealbreaker since Lagertha values being an independent ruler rather than a Queen to a King. It's the same reason why she refuses to wed Kalf earlier in the series. It's a shame that the pairing doesn't last long because the two were slowly becoming one of the best Vikings couples.
Ragnar Lothbrok
Lagertha and Ragnar are happily married at the start of the show. They remain a happy couple until Ragnar proposes bringing in Alsaug as a second wife. Lagertha rejects the idea and so the two divorce.
Ragnar is portrayed as the most reasonable one among all of Lagertha's lovers. He is honest with her about almost everything, even eventually coming clean about his cheating with Aslaug, which they would have worked through had she not shown up pregnant. When the two part ways, they still care about each other. Lagertha is willing to spend time away from her husband in order to help Ragnar fight his battles. The two also fight together in the Battle for Paris. The fact Ragnar is also the only person Lagertha ever has a lengthy relationship with says a lot.