Forget the flat tax.
Our tax system started out as a flat tax, although you could argue there were two tax brackets--0 and 10%, as the tax was only levied on the wealthy. After the system had grown into a pyramid with rates as high as 90%, in the 80's Reagan's reform flattened it to only 3 brackets. It has now grown back to, what, 6 brackets? With a surtax here and an AMT there, etc.
Flat taxes sound nice, but don't work. They don't stay flat.
The Fair Tax makes it none of the Government's business how much money I make, or how much I spend. So two words: Fair Tax.