Which is better: to perform Hajj on behalf of deceased parents who had already fulfilled their obligatory Hajj, or to spend the money on building mosques and striving in the path of Allah?

The best way to honor one's parents after their passing is to supplicate for them, seek forgiveness for them, honor their friends, and maintain kinship ties that are connected to you only through them. As for performing Hajj, offering sacrifices, or giving charity on their behalf, these are permissible but less virtuous, and supplication is better than them. The righteous deeds intended to be gifted to parents should rather be performed for oneself, because a person is most in need of them. The Prophet, peace and blessings be upon him, guided to what is best and most beneficial when he said: "When a person dies, his deeds are cut off except for three: ongoing charity, knowledge from which benefit is derived, or a righteous child who supplicates for him." He did not mention anything other than supplication from the children's deeds for their parents.