I may have already mentioned this at the beginning of this thread, but I'll explain my reasons:
Inspector Gadget
Like Mr. Marshmallow said, this is probable one of the greatest theme songs ever, if not the greatest cartoon theme song ever. A classic show should have a classic theme, and Inspector Gadget is no exception!
Upon listening to it, you already know that it's a classic theme, and it works really well with the opening animation. I could explain it a bit more, but that's all to say about this. A brilliant, well-made theme!