Viking 1
Viking 1 was the first of two spacecraft (along with "Viking 2") sent to Mars as part of NASA's Viking program. On July 20, 1976, it became the second spacecraft to soft-land on Mars, and the first soft lander to successfully perform its mission. (The first spacecraft to soft-land on Mars was the Soviet Union's Mars 3 on December 2, 1971, which stopped transmitting after 14.5 seconds.) "Viking 1" held the record for the longest Mars surface mission of days (over 6¼ years) or Martian solar days, until that record was broken by "Opportunity" on May 19, 2010.
NASA
Outside Mars, Jupiter was first visited by "Pioneer 10" in 1973. More than 20 years later "Galileo" sent a probe into the planet's atmosphere, and became the first spacecraft to orbit the planet. "Pioneer 11" became the first spacecraft to visit Saturn in 1979, with "Voyager 2" making the first (and so far only) visits to Uranus and Neptune in 1986 and 1989, respectively. The first spacecraft to leave the solar system was "Pioneer 10" in 1983. For a time it was the most distant spacecraft, but it has since been surpassed by both "Voyager 1" and "Voyager 2".