In my honest opinion... Museums are a waste of time. They don't hold any value and usually don't contribute to the story line. I guess the Oceanic Music was decent because I found some of it interesting. So yeah maybe they aren't useless.
My favorite museum would have to be in Lilycove. I loved how the manager would reward your effort in the contests with pictures of your pokemon, and how more and more people would come to see your paintings. Really gave a sense of accomplishment.
The Oceanic Museum was my favorite. When I was younger, I used to constantly visit the museum, look at the ships, and read their descriptions. Haha. I kind of liked the Lilycove Museum, too. The paintings were quite interesting to look at and I loved how they allowed you to display pictures of your Pokémon there, just like most of you. Also, their music always made me feel calm. <3
I always liked Lilycove's Museum slightly better, probably because I actually had a Pokemon narrowly win a Master Beauty rank contest, which is a Roselia. Still remains the only star contest performer I ever used in the game but I like how you can get your star Pokemon a painting if they win a Master Rank contest.