Frigga loves both her sons equally. She will always see them as her children, even if you take the myths as canon for the films, which means that Thor is not her biological child anymore than Loki is. Taking that into consideration, that makes Frigga an amazing mother, and an even better person.
Who among us can honestly say that we, in a situation like Frigga’s, where you are asked to raise first the child of your husband and another woman, and then the child of your greatest enemy, would actually accept this job? I can honestly say that I don’t really know what I would do in that situation. Frigga, however, looked at these two children who needed her, and said that she would raise them.
Now we don’t know if Thor is Frigga’s actual son or not in the canon of the films. But if he is, then that does not lessen her actions in taking Loki as her own at all. If anything, it makes her even better. She would have a child of her own with her husband, who would eventually become king after Odin. She had every right to turn away when Loki was brought to her as a child. Remember that Loki is the son of Laufey, the Jotun that Odin had been fighting for we don’t really know how long. Frigga would have been fully in her rights as queen and a mother to tell Odin how full of it he was when he asked her to raise this abandoned child.
She is my hero forever, because she loved Loki wholeheartedly. Her love for Loki was so great that, even when he denied Odin and Thor (who both also loved him, even though Odin did not show it in the best manner) he could never truly deny Frigga. Even when he tells her that she is not his mother, both she and the audience can see the lie in his words.
At the end of the day, though, when there is no violence between her sons to fret about and nothing but being a wife, queen, and mother, Frigga does not love Loki better than Thor or vice versa. They are hers, both of them. Neither is the golden son over the other, and neither is the lesser son.