One of my favorite NGM issues on space has to be from August of 1970: The Grand Tour of the outer planets as it was envisioned before budget cuts and scaled down to the Voyager probes.

Along with great artwork from the late Ludek Pesek, it is still amazing to read how much we did not know about the Solar System even after man landed on the Moon. One big example - Mars was considered to be a flat and uninteresting place. If only Mariners 4, 6, and 7 had taken images of even just one channel or volcano, perhaps we would have had manned missions there in the 1980s as originally envisioned.

The issue also contains a map of the constellations and a smaller article on the total solar eclipse of May 7, 1970, the first one I ever remember, though did not directly see.