The most important part of the tax cut Ronald Reagan has proposed may turn out to be the one that has been least debated: the business tax cut, which would reduce corporate tax liabilities by nearly half in the next five years.

Reagan would liberalize depreciation rules, the speed at which businesses can deduct the cost of new buildings and equipment.By 1986 this would save corporations $60 billion, equivalent to about 6 1/2 percent of projected total spending, or 10 times the amount Reagan plans to spend on food stamps that year.

Reagan and his advisers argue that such a cut in business taxes would pay important dividends, in that it would stimulate investment and help considerably in reviving the economy.

Moreover, although its eventual cost would be high, the plan starts off deceptively cheaply. In fiscal 1982 it would cost only $9.7 billion, according to administration estimates, even if it is backdated as proposed to take effect on Jan. 1 of this year.

But even proponents acknowledge there are some problems with the plan.

For one thing, it would mainly help the strong rather than the weak. In order to benefit from write-offs against tax, companies have to be paying tax in the first place. The profitable oil companies would be big winners from the 10-5-3 plan (so called because all investments would be written off in 10, 5 or 3 years). The money-losing auto, steel, railroad and airline companies would not be directly helped at all.

In general, unprofitable or labor-intensive firms, small businesses with lower investment levels, high technology companies where ideas rather than new machinery are what matters, and interest-squeezed savings and loan associations that want more incentives for savers rather than for investment would all do better with some other tax cut measures.

A second important problem is that the plan outstrips its rationale. By many experts' reckoning, when combined with the investment tax credit, it would in many cases wipe out all taxes on income from new investment, and in some cases even shelter additional income from tax.

The heart of the 10-5-3 scheme is that almost all investment in machinery and equipment would fall into a five-year category for depreciation, and would be eligible for the full 10 percent investment tax credit as well. Under current law, machinery and equipment can be written off in from three to 36 years, and only investments written off in eight or more years qualify for the full investment credit.

Because so many firms would gain so much from this provision, those that would gain less or not at all have not spoken up in opposition.

But most are ready with suggestions when and if a propitious time arrives.

One suggestion backed by some of the money-losers is to make the investment tax credit refundable, and so have the Internal Revenue Service sending checks to unprofitable industries who invest.

But there is now only one refundable credit on the books -- the earned income tax credit for the working poor with children -- and there is great resistance in Congress to enacting such an obvious subsidy or "handout" for business.

Business tax lobbyist Charls E. Walker, who numbers some of the basic industries such as autos and steel among his clients, says some people prefer the notion of "transferability" of the credit. Companies which cannot use their tax credits should be allowed to sell them to those who can, he argues. In that case the cash flowing into the unprofitable companies would come from other firms, rather than directly from the government, Walker reasons.

Chairman Dan Rostenkowski (D-Ill.) of the House Ways and Means Committee has suggested another alternative: allowing companies to "carry back" more of their investment tax credits for longer than now allowed. Under this scheme they would still get checks from the IRS, but these would "rebates" of past taxes they had paid.

High technology firms have also been campaigning quietly but effectively for special help. The frontrunner at the moment is a special 25 percent investment tax credit for research and development expenditure.

Ironically, the administration's proposal for encouraging R&D -- allowing investment in R&D equipment to be written off in three rather than five years -- would hurt rather than help, according to tax experts. The smaller investment tax credit allowed equipment in the three-year category under the 10-5-3 proposal would cost companies more than the depreciation speed-up would gain them.

Norman Ture, treasury assistant secretary for tax policy, said last week he had not heard complaints of this, but would be willing to look into it if necessary.

A spokesman for the American Electronics Association commented that while 10-5-3 is "important to the nation's economy and business," the association has "an idea or two peeking out of our sleeve" about how to help high technology firms in particular.

Small business groups are backing the president's plan largely because of the great simplification it would entail in their tax calculations. Better for their cash flow would be a cut in the corporate tax rate, or a special lower rate for small firms.

Some congressmen support such a measure as part of a business tax cut. It was in the tax bill reported out by the Senate Finance Committee last year, and as Walker has noted, that bill reflected many still-existing political pressures. But if the administration keeps pushing for tax cuts linked to new investment, then a corporate rate cut is less likely.

The savings and loan industry would like to have at least some of the money available for tax cutting this year spent on particular savings incentives. If this were a normal year, some measures to encourage savings, such as a liberalization of rules for individual retirement accounts, would be bound for inclusion in a major tax bill.Republicans have long supported saving incentives, and Rostenkowski gave them his blessing last month.

But since any additional measures would entail taking money away from the main two components of the president's tax plan -- 10-5-3 and his proposed individual tax cuts -- the key could be the extent to which the administration is prepared to compromise on the bill.

Changes in the treatment of structures in the president's plan are widely expected. At present, the bill before Congress would reverse the present bias in favor of residential over nonresidential building, and would to some extent offset that of leased over owner-occupied buildings. The real estate industry has complained about the discrimination, and experts now believe it could be difficult to distinguish for tax purposes between owner-occupied and leased property in some cases.

Another change that the Treasury is likely to recommend, after vigorous lobbying by some groups, is an improvement in the tax treatement of foreign assets. The Reagan plan would leave them being taxed more heavily than they are now.