Imo, Plantera is the worst boss.
All she does in the fight is follow you and this makes me feel like she was intended purely for catching you off guard in small caverns with her bulbs because the moment you get even the simplest of arenas you, or at least me, can fall asleep just circling her. In addition, I think her theme is overrated - it's not necessarily bad but I don't see why it's so good.