The Soviet probe that first reached the surface of another planet in 1966 was Venera 3.
Venera 3 was launched by the Soviet Union on November 16, 1965, on a mission to Venus. Its entry probe was designed to descend through the Venusian atmosphere and reach the surface while the accompanying carrier spacecraft provided communications and navigational support.
The probe reached Venus on March 1, 1966, making Venera 3 the first human-made spacecraft to hit the surface of another planet. However, it did not complete a successful landing or return usable scientific data: contact had been lost before arrival, and the entry vehicle crashed into Venus. That distinction matters when comparing early planetary milestones.
Venera 7, another Soviet mission, achieved the first successful soft landing on another planet and transmitted data from Venus in 1970. Mars 3 made the first soft landing on Mars in 1971, but its transmitter stopped working shortly afterward. Luna 9 reached the Moon in 1966, but the Moon is Earth’s natural satellite rather than another planet.