What is the largest single bequest to the Met, consisting of over 3,000 works?

The largest single bequest described in this question is the Robert Lehman Collection.

Robert Lehman, the investment banker and longtime Met trustee, built on a collection begun by his parents and expanded it over roughly six decades. After his death in 1969, the collection passed to The Met through the Robert Lehman Foundation. The museum accepted Lehman’s wish that the works remain displayed together, and the Robert Lehman Wing opened to the public in 1975.

The collection is broad rather than confined to one period. It includes Italian Renaissance paintings, Northern European and Spanish works, Old Master drawings, manuscript illumination, sculpture, glass, textiles, maiolica, enamels, and antique frames. Particularly notable are its Sienese paintings, Venetian drawings, Renaissance ceramics, and European decorative arts.

Published totals vary because sources count the bequest differently: The Met identifies about 2,600 works, while some popular accounts round the figure to nearly 3,000 or describe the wider family holdings. That numerical variation does not change the canonical identification. The other named collections were important Met gifts, but none matches this distinctive unified bequest and dedicated wing.