I would imagine that besides "pushing the final frontier" it would help if each of the six spacecraft represented different types of spacecraft, allowing each lecture to cover different aspects of space exploration. My picks would be:
Sputnik 2 - 1957
The second satellite in history, Sputnik 2 was the first to carry scientific instruments, making the first direct scientific observations from space. It also carried a dog named Laika which was the first living organism launched from Earth into orbit, and the first mammal. Although Sputnik 1's radio transmitter provided some limited scientific information, Sputnik 2 really marks the beginning of scientific exploration of the cosmos by spacecraft.
Vostok - 1961
Placing the first human into outer space cannot be minimized in terms of the historic achievement. And while Sputnik had a big effect on the space race, what really created the push to reach the Moon was Yuri Gagarin's Vostok flight. With some modifications, the Vostok (East) capsule, renamed as Voskhod (Sunrise) enabled the world's first spacewalk, the first (relatively) soft return landing on dry land, and the first space flights with two and three people. And the world's first female astronaut Valentina Tereshkova flew on a Voskhod capsule in 1963.
Apollo Lunar Module - 1969
It was the first spaceship built solely for flying in space. And the first, and so far only spacecraft to land astronauts on another world.
Space Shuttle - 1981
The first reusable spacecraft, the first spacecraft recovered from orbit intact, the first spacecraft to carry four, five, six, seven and eight passengers. Essentially the first work vehicle in space, the Space Shuttle was equipped with a robotic arm, massive cargo bay, and a built-in airlock, enabling transporting astronauts and equipment to and from a worksite and supporting their on-orbit operations. Although famously expensive to operate, the Shuttle had a profound effect on our perceptions about what is possible in human spaceflight, as well as highlighting the dangerous and unforgiving nature of space travel with tragic clarity.
Voyager 1 and 2 - 1977
Not the first interplanetary space probes, but so far the ones that have gone the farthest. Made the first ever visits to Uranus and Neptune. And they are the first spacecraft to leave the solar system, or at least explore its boundaries. After nearly fifty years the Voyager spacecraft are still transmitting data. Voyager 1 is currently more than 160 times further from the Sun than we are. The radio signals that it transmits, travelling at the speed of light, take nearly 24 hours to reach us.
Hubble Space Telescope - 1990
Not the first space telescope, but because of its capabilities Hubble seemed to mark the beginning of an unprecedented period of discovery, as well as captivating an entire generation with its stunning visible light views of the universe. It is also so far the only space telescope designed to be maintained on orbit by astronauts. Like the Apollo Moon landings, Hubble's impact our perception of space is hard to measure, but clearly massive.