2001: A Space Odyssey comes top of every list, but the sequels are not rated as highly by many people.
Second on the lists is Rendevous with Rama.
At first, only a few things are known about the celestial object that astronomers dub Rama. It is huge, weighing more than ten trillion tons. And it is hurtling through the solar system at an inconceivable speed. Then a space probe confirms the unthinkable: Rama is no natural object. It is, incredibly, an interstellar spacecraft. Space explorers and planet-bound scientists alike prepare for mankind's first encounter with alien intelligence. It will kindle their wildest dreams... and fan their darkest fears. For no one knows who the Ramans are or why they have come. And now the moment of rendezvous awaits — just behind a Raman airlock door.
The strange thing with this book is that we never meet the aliens, and nothing really happens. It is all about the exploration, with wonderfully detailed descriptions of what they find inside the giant rock.
Like 2001 this also has sequels with mixed reviews.
Third is Childhood’s End
Without warning, giant silver ships from deep space appear in the skies above every major city on Earth. Manned by the Overlords, in fifty years, they eliminate ignorance, disease, and poverty. Then this golden age ends--and then the age of Mankind begins....
It turns out that this elimination leaves mankind without ambition, bored. But the Overlords are waiting for the children, who develop abilities that are the next stage in evolution.
SyFy channel had a three-part miniseries which inevitably changed some things but was quite well received and brought Clarke’s work to a new audience.
The City and the Stars
A billion years into the future, Earth’s oceans have evaporated—and humanity has all but vanished. The inhabitants of the City of Diaspar believe theirs is the last city—but there is no way to find out for sure. The city is completely closed off by a high wall, and nobody has left in millions of years.
The last child born in the city in millions of years, Alvin is insatiably curious about the outside world. He embarks on a quest that leads him to discover the truth about the city and humanity’s history—as well as its future.
The Fountains of Paradise
Vannemar Morgan's dream of linking Earth with the stars requires a 24,000-mile-high space elevator. But first he must solve a million technical, political, and economic problems while allaying the wrath of God.
The first two-thirds of the book are actually about Morgan’s struggle to get permission and a location to build the space elevator. It is only in the last third that the elevator actually gets built.
Ann Marie Thomas is the author of three medieval history books, a surprisingly cheerful poetry collection about her 2010 stroke, and the science fiction series Flight of the Kestrel. Book one, Intruders, is out now. Follow her at http://eepurl.com/bbOsyz