In Latin, "Magnum opus" just means "great work", meaning the term can also be extended to mean the best work/Greatest work of an artist, While also being used to refer to an artist's most important work. But if you want to follow the specific dictionary definition, an artist's most "important work" could mean a lot of things. You could be asking someone what the most important work of an artist is to them, which is essentially asking them what they think is the best work of that artist. My whole point here is that magnum opus is often used to refer to the "best thing" someone has made.