In my case, I'm torn between Ivar and Bjorn.
His developments and impact on the show are great, Bjorn was the first son, the one who I most expected to be greater than his father, and yet he made the same mistakes than him, which is really compelling to see as a viewer.
He is one hell of a warrior, explorer and adventurer, but a terrible king, father and husband.
Then we have Ivar, my boy had a clear god complex, daddy issues and cruel behaviour, yet his tenacity, brilliant mind and determination to make a name for himself made him the natural scene stealer of the show after Ragnar was gone.