I would have to rate Mariner 4, which showed us what the surface of Mars REALLY looked like, the Hubble Space Telescope, which has revolutionized astronomy and our understanding of the most distant universe and black holes; and the Voyager spacecraft which gave us our first good looks at the outer planets and many of their moons. The Venus Magellan satellite also deserves high praise for its mapping of the surface of Venus.
These are my candidates...but no list is complete and everyone has their favorites.