If the number of drop-in visitors is the criterion, surely the most popular federal agency in Washington yesterday was the Internal Revenue Service. From the time its local tax office at 1201 E St. NW opened until its closing last night, a constant line of people streamed in, seeking various forms and instruction booklets to help them meet tonight's filing deadline.

The forms normally are available only on the eighth floor, but the building's three elevators became so jammed that cartons of the most-sought forms were spread along the lobby floor. Only those seeking more specialized forms--or assistance in preparing returns--had to go upstairs.

Among the elevator travelers was Gert Stern, from Great Britain, a World Bank employe who lives in McLean. In his first visit to a U.S. tax office, he said he was "exploring the need" to pay taxes on some investments.

The office, and its suburban counterparts near Baileys Crossroads in Virginia and at Wheaton Plaza and Suitland in Maryland, will be open from 8:30 a.m. to 4:45 p.m. today. Midnight, of course, is the deadline for mailing federal as well as Maryland and District of Columbia income tax returns. Virginians have until May 1 to file state returns.