I always liked the basic income plus flat tax combination - simple and fair. Due to the means testing of various aid programs the tax system if often regressive and always confusing for low income earners. Exactly the opposite of what we should all want.
http://en.wikipedia.org/wiki/Negative_income_tax#Flat_tax_wi...
Edited to add: It's worth noting, though, that you can't produce every possible progressive tax rate curve with this method. It's certainly possible to prefer some curve that is unattainable this way. But a simplistic "a tax flat in dollars is regressive in utility" isn't applicable.