What was the first planet that has been landed on?

The first (non-Earth) planet landed on (i.e., a soft landing) by a man-made spacecraft was Venus, by the Venera 7 probe from the USSR in 1970. The parachute seemed to fail on descent, precipitating a harder landing than intended, and the craft likely rolled onto its side resulting in a bad antenna aim towards Earth - but some weak signals were received, from which at least the surface temperature could be obtained.
There were of course earlier soft landings on the Lunar surface (but this does not qualify under modern definition of a planet).