In my mind, the "fairest" approach would be to have a big enough VAT or consumption tax on all goods and services that are consumed, in order to support the needed level of government spending. Meanwhile, eliminate all of the income tax code. In order to make it progressive, everyone would receive a monthly or annual transfer-payment of some multiple of the poverty rate, to make up for paying any of the consumption taxes on some base-line standard of living.
It's not perfect, but it would eliminate a huge corrupt income-tax-code that has a million different carve-outs for special interests. It would tax anyone who wanted to consume anything. So if a wealthy person wanted to be a saver/investor, they could choose to do that instead of buying yachts.
Of course, this would never happen. Too many businesses and people's jobs depend on having this byzantine federal tax code and the need to earn an income by helping or forcing everyone to comply with it. It would also neuter a lot of the ability for congress/reps to take bribers...
You've got two guys. A has 1,000$ to his name, and B is worth 1,000,000,000.
They both have to eat. So A buys a bag of rice and a bag of beans . . . and incredibly manages to get the total cost of his meals down to 1$ per day. B only eats sushi off naked women, so he's spending about 10,000$ a day on food.
They both have to live somewhere. So A has rented a cockroach infested place with five other guys and only has to pay 500$ a month. B has three mansions that he bought in the past that he jets to and from. He's paying 30,000$ each month in property taxes on the mansions.
They both need clothing. So A will occasionally buy used stuff from Goodwill. Let's say 5$ a month, because this shit is already pretty worn so it wears out. B likes to buy clothes that he'll never wear and then burn them for fun. So 15,000$ a month.
They both need to get around. A has a monthly bus pass for 120$. B likes to travel by helicopter . . . so let's say 50,000$ a month all in.
So let's add up monthly taxes, assuming a flat rate of 5% for purchases:
A
Food - 1.5
Rent - 25
Clothes - .25
Transportation - 6
Total - 32.75
Which gives a yearly total of - 393$
B
Food - 15,000
Rent Property Tax - 30,000 (yeah, it's not a consumption tax . . . but rich people always own property, so let's just count it that way)
Clothes - 1,500$
Transportation - 2,500$
Total - 49,000$
Which gives a yearly total of 588,000$
Looks good so far right? The extravagant B is paying way more in taxes! Well . . . when you look at how much they're paying based upon net worth (ignoring all frivolous spending - just looking at the essentials necessary to survive here) . . . A is spending
39.3% of his net worth in taxes each year just to survive and B is spending
0.0588% of his while being a wasteful dick.
Consumption taxes tend to be rather regressive. They're great if you're rich enough that the cost of survival is so low you can choose to consume or not to. But most people dismiss them outright because of how unfairly they penalize the poorest folks in society.