Coming from a historical perspective, it makes sense we haven't seen many women on the battlefield, but this manga definitely suffers from a lack of interesting female characters. Helga and Ylfa can be entertaining but there isn't much to them as characters. Arneis was better written than the previous females in the story but she didn't last long. Gudrid, even though she just showed up, is easily my favorite already. She has clearly been set up with room to grow as a character (hopefully into someone interesting) probably in some attempt to finally address the gender issue this manga has.
I was thinking about the history, though it would be predominantly male what about the shield maidens? They haven't been mentioned at all, and in Vinland saga it seems they are supposed to just be homemakers/supportive mothers and wives.
I voted for Ylfa cause she seemed to be an antithesis to the traditional female presented in Vinland saga and seems more willing to speak her mind. However you are right Helga and Ylfa are minor side characters, Arneis was written with a lot more depth and her end was very moving. Gudrid might become a major character and is rebelling/questioning against the norms of Vinland Saga's society and what it expects of her. Also her barrel disguise and arguments with Leif are adorable. She seems to be based on Gudrid Thorbjarnardóttir, and so I expect she will somehow accompany Thorfinn to Vinland.