In the notoriously fickle world of Wall Street, Edward S. Hyman is a conspicuous exception.

The slight, amiable 45-year-old, who resembles Judge David Souter (but with a closer shave), has survived bull markets, bear markets and chicken markets to win enduring recognition as the financial community's favorite and most reliable economist. His colleagues have so designated him an astounding 11 consecutive times in the annual Institutional Investor poll.So what Hyman thinks about the economy is of more than the usual significance, and what he thinks right now turns out to be characteristically balanced and moderate. He sees the longest peacetime expansion in American history slowing to a possibly fatal limp. But he doesn't foresee the kind of extreme hard times ahead that have lately been spooking the financial markets - and producing a new round of far-out gloom-and-doom speculation.

Hyman, who is vice chairman of C.J. Lawrence, Morgan Grenfell Inc., is no Pollyanna. His best guess, he told me, is that the U.S. economy "is going to be flat for at least a year."

That's not good news for anyone seeking a job - or even just a better standard of living. Hyman expects unemployment to rise from the pres-ent 5.5 percent range to 6.5-7 percent by next summer, and he warns that the oil shocks emanating from the Middle East will continue to punish consumer prices for the next two or three months.

But he is surprisingly optimistic about our ability to contain inflation in 1991 (he's forecasting only a 3 percent increase, which would be a dramatically heartening turnaround), and he's downright bullish on the recently battered American dollar. He's predicting a healthy snapback for the greenback, even though he sees U.S. interest rates coming down sharply as the economy slows.

As far as investments are concerned, this means Hyman's No. 1 recommendation plainly is bonds. He thinks the yields on long-term U.S. Treasury securities, which have spiked above 9 percent amid growing Mideast tensions, will tumble to 7.5 percent by next summer.

If Hyman is right in his forthright prediction that "interest rates are peaking now," this suggests important implications for ordinary consumers, too. This would be the time to lock in longer-term certificates of deposit, for example, and to begin thinking a bit more aggressively about home ownership.

But he believes stocks will be less attractive than bonds for the next few months, as equities continue to reflect uncertainties about interest rates, the price of oil and corporate earnings - presumably until stock traders begin to look beyond the slowdown, to the resumption of economic growth Hyman foresees in 1992.

Forecasting is easy; accurate forecasting is hard. And a knife's-edge forecast like Hyman's - for an economy poised precisely between slump and expansion for as much as a year - is clearly subject to accidents along the way. He feels such accidents are more likely to push us into mild recession than renewed growth, but he sticks to his "flat" forecast as the most probable outcome.

On the negative side, he cites familiar woes: the hangover from excessive public and private debt creation in the 1980s, the Mideast problems, the pressure placed on U.S. monetary authorities by surging interest rates abroad, the elephan-tine costs of the savings-and-loan bail-out, the fear that bank regulators may be inducing a credit crunch.

But he regards these as only half the picture and also discerns some important economic strengths: short-term interest rates remain below long-term rates (the reverse situation, known as a "negative yield curve," often portends recession), excessive buildup of inventories has been avoided in this computerized age, there is no capital-spending boom that needs to be busted, housing and auto sales are already down at recessionlike levels, surprisingly strong American exports add to our fundamental outlook.

In an arena where wild, far-out predictions - the scarier the better - too often garner the headlines, a sober but not panicky view like Hyman's may seem too sensible to command attention. But it's just such balancing of all the factors, rather than racing to one extreme or the other, than has built this still-youthful economist's remarkable reputation. He could be right again.