Belphegor. My friend used to listen to them in high school a lot when I was just getting into metal, and 10 years later I started giving them a go. Now I’ve been listening to them with some regularity and have some favorite songs, which also show up in my rotation regularly.
The Beatles
Didn’t dislike them as artists, but when The Haunted released The Dead Eye, I hated it. But I kept listening and it became one of my favorites.
Black metal as a genre. I could not believe that anyone seriously listened to that. It was either random noise or actually scary stuff (I am the black wizards actually scared me for some reason lol). But after enough exposure and playing Freezing moon myself, I actually got to enjoy it to the point where now I listend to black metal 80% of the time.
In the same vein, “Sons of Winter and Stars” by Wintersun was so unharmonized and terrible to me when I first listened to it, but after a few listens it became one of my favourite songs
I couldn’t stand Linkin Park when they were first getting popular. I felt that they were too edgelord-y for me and wrote them off. Now, I wish I had actually given them a shot earlier on.