Flat.

The poverty line in the US is about $24k, I think. I would be fine with a flat tax of 5% below the poverty line (everyone should have skin in the game) and an additional 1% for every 1K earned up to 20% total, which would be reached at roughly $40k. The natural response from opponents is to say that its not fair because someone at $40k will have to pay $8,000 which has a greater impact than someone at $200k having to pay $40k. To that I say, "that person paying $40k payed 5 times more to the government to keep our country running. Also, $40k is a lot, even if you are still taking home $160k.

People that want to excessively tax the wealthy should try and start a business one day. It's hard. VERY, VERY hard. Most people toil away at it for many years before they're successful. Then, if they're lucky they progressively climb the income ladder. Eventually, after missing out on years of life to build a business/service that presumably has value for society, you want to tax them at 90%? That's just crazy.

Flat tax. It's fair. The wealthy will pay more than they are right now and its unambiguous.

Edit: Was just reading an article with this breakdown of GOP candidates proposed tax: