Wish we could select multiple!
Boss 3 (Destroyer, BoC) was my childhood favourite (always used to hum it in school lmao), though nowadays I like most of them, but ML's theme probably takes the first place because of the chaotic and immense feel that gives you the intended "final fight" vibes. Fan-favourite Plantera is up there too, probably around 2nd or 3rd place for me.