Interesting note, as of today, the US is the first country to have successful spacecraft missions visit all of the classical planets.

There is one big exception, of course: the Soviet Union was the first to have a spacecraft visit Earth, with Spunink.

This category is completely arbitrary, especially given that Pluto is now considered a minor planet. Still, it's an achievement of note.

  Mercury - 1974, Mariner 10

  Venus --- 1962, Mariner 2

  Earth --- 1957, Sputnik 1

  Mars* --- 1965, Mariner 4

  Jupiter - 1973, Pioneer 10

  Saturn -- 1979, Pioneer 11

  Uranus -- 1986, Voyager 2

  Neptune - 1989, Voyager 2

  Pluto --- 2015, New Horizons

* The Soviet Union launched several probes to Mars before the US did, but those probes failed. This may be true of other planets as well.

edit: formatting