A great artist makes things that are exceptionally unique and thought-provoking. A successful artist makes things that have wide enough appeal to generate sustainable income. Whether the piece is fun is irrelevant to the caliber of an artist, I think. Some artists toil night and day for finished products that mean nothing to anyone but them.

ideally both, but with a preferential disposition towards the former. I see no reason why it has to be one or the other. false dichotomy etc.

Not just artists. This is relevant for probably anyone with multiple successful side projects.

Finding stuff at the intersection of the set of things fun to make and the things other people will enjoy is the hardest part of deciding what project to work on. It's a skill. You need to have a lot of ideas to find one landing in this sliver.