IF FEDERAL TAXES must be raised--and it's beyond dispute that they must--the question is which taxes. The widespread hostility to the income tax is a matter for real concern. The Advisory Council on Intergovernmental Relations has just published the results of its annual poll on American taxation and, as in each of the previous four years, the respondents voted the income tax to be the least fair. The runner-up, as usual, was the local property tax. The poll then asked its respondents to choose between higher income taxes and a federal sales tax on all purchases except food. By more than two to one, the people polled said that they would prefer the sales tax. If that truly represents national opinion, it shows an astonishing swing over the years.

But like many polls, this one contains contradictory currents. Asked how to make the national tax system more fair, respondents most frequently answered: make upper income taxpayers pay more. A sales tax won't do that. Again, asked how best to increase income taxes, respondents most frequently favored cutting back on itemized deductions like those for state taxes and mortgage interest.

Over the past decade, the proportions of people who consider the income tax to be the most unfair has risen and fallen in close correspondence with the inflation rate. In 1972 the income tax's rating for unfairness was low; it leaped upward the following year when the inflation rate doubled, and leaped again with the inflation rate in 1979. Since public resentment of inflation and its efficiency in kicking people into higher tax brackets helped elect Ronald Reagan president, it is not entirely surprising to see the same sentiment reappear here.

Polls don't reliably tell politicans how to treat people fairly, least of all in a subject as complex as taxation. You are entitled to wonder whether the people being polled here are fully aware of the implications of a national sales tax. But polls do tell politicians accurately whether people feel that they are being treated fairly.

The great irony conveyed by the recent ACIR polls is that there has never been a tax law so intricately adjusted and amended and revised in the name of fairness to one category of taxpayers or another as the income tax. After 70 years of it the result is a tax so riddled and intricate that a great many Americans now consider it the least fair tax of all. The poll is probably giving Congress good advice when it suggests that most people's idea of fairness indicates both neutrality toward inflation, and simplicity.