SCOTUS ruled that the Affordable Care Act was a tax. A regressive tax is one that disproportionately taxes people's income at a higher rate for people with lower income than people with higher incomes.
Is the ACA a regressive tax?
Which income quintiles are most affected?
The 'tax' part only kicks in if you a) don't have insurance and b) decide not to buy insurance. So that makes this a bit trickier to answer as we have to make assumptions as to who will decide not to have health insurance.
But let's ignore that part.
If we treat it merely as a purchase one has to make, we could say it's "regressive" in the same way that buying a can of soda is "regressive". For someone that earns less money, more of their income percentage-wise has to go towards that purchase.
So, the less money you make, the more "regressive" this purchase will be.
That would be true down to the point where you meet the subsidy thresholds, which are between 100 and 400 percent of federal poverty levels. At that point, you could argue the purchase is now "progressive" in comparison.
To answer the specific questions literally:
Is the ACA a regressive tax?
No, it's not a tax. There is a penalty component, which is considered a tax by the SCOTUS.
Which income quintiles are most affected?
Hard to say with any certainty because the penalty is not tied to income, but whether or not a person chooses to avoid having a health care policy. Once we see who makes those decisions, we can then determine if they fit into particular income demographics.
It should be noted, however, that the penalty is tied to income above a certain threshold, so one would likely not consider the penalty, in and of itself, regressive.
Summary: One could argue that the concept of having to purchase insurance is regressive through a particular range of the income spectrum. The penalty for not purchasing insurance is not (again through a particular range of the income spectrum).
I found a Congressional Budget Office report that should answer this question. The CBO report makes predictions as to whom will be paying penalties for not having insurance, and how much they will be paying. It notes that most people left uninsured will not be subject to the individual mandate, and will not have to pay a penalty. However, among those who do pay the penalty, most will be above the median household income, and most payment will come from them.
Here is a table of the CBO's predictions as to whom will be paying the penalty and how much they will be paying, broken down by income level.
The median household pays the penalty will make around 300% of the federal poverty level. Currently, the federal poverty level for a household of four is $23,550, which means that a household of four at 300% of the poverty level would be making $70,650, which is more than median household income ($51,017).
The CBO predicts that 77% of payment for the penalty will come from people making more than 300% of the federal poverty line, who comprise 49% of those paying the penalty. Those with higher incomes will end up paying proportionally more than those with lower incomes.
