Flat tax is bullshit, for painfully obvious reasons. Perfectly, taxes would be levied from amounts over cost of living and stuff. Otherwise, you'd have people below the poverty line paying tax.
Corporations should pay taxes legally, but they often don't. I agree that corporations raise prices to comply with tax requirements (even though a lot don't pay taxes), and that the poor are unfairly targeted by high prices on stuff.
That doesn't mean the only solution is to reduce corporate taxes, or to remove them. It may well be the best solution, though. Curiously, I can find no obvious flaw in krellin's reasoning. A no-loophole scaling income tax, with a tax on investments could lead to an interesting situation.
My perfect tax is indeed flat, but with a flat numeric tax reduction for everyone. This means you can have a reasonably high tax, while poor people don't lose most of their money, and the horribly rich pay reasonable rates, but that's never gonna happen.