Depends. Who has the least currency?
Let us first take a look at the economics of inflation.
A certain economy has \$100 in circulation. If the government inflates the currency with an additional \$100, there is going to be \$200 in circulation.
If a crate of apples originally cost \$1 in the initial economy, the market will adjust the cost to \$2 in the inflated economy.
The original dollar only has half the purchasing power (and is only worth half a crate of apples) because the total amount of money in the economy doubled.
The same principles from this simple example can be applied to any real economy. When the government injects more money into the economy, each piece of currency in the economy loses a portion of its purchasing power, and the market adjusts by increasing the prices of goods and services.
Inflation is no respecter of persons. Bill Gate's money has devalued at the same rate as mine.
So, with respect to inflation, the currency is taxed neither at a progressive nor at a regressive rate; the currency is taxed equally.
However, notice that the taxation caused by inflation affects the currency only.
Let us return to our example.
If we purchased a crate of apples in the \$100 economy, we would only pay \$1.
Then the government inflates the economy to \$200. If we try to resell the crate of apples (assuming that they have not spoiled), we can expect the selling price to be \$2.
There is nothing special about pre- or post- inflation apples; the apples are still worth the same amount and require payment with an equivalent purchasing power.
Now, we head back to the real world.
When people purchase books, cars, refrigerators, vacation homes, stocks, bonds, etc., they are shielded from inflation. They can sell the stocks, bonds, and homes at the new inflated price.
Unfortunately, whoever took their money suffers the brunt of inflation and is poorer for holding onto their money.