osmium

In 2002, it was shown to be the least compressible substance known, with a bulk modulus of 462 gigapascals.¹

1. Hyunchae Cynn, John E. Klepeis, Choong-Shik Yoo, and David A. Young.
Osmium has the lowest experimentally determined compressibility.
Physical Review Letters, volume 88, page 135701, 1 April 2002.