I also like 'The Sentinel'. This story was the inspiration for '2001: A Space Odyssey'. It's a very short but powerful story that sets the stage for the exploration of extraterrestrial intelligence and the mystery of the monoliths in the later work.
Another great one is 'A Fall of Moondust'. It takes place on the moon and has a really interesting setting. The plot is full of twists and turns as the characters face the challenges of being trapped in moondust. It shows Clarke's great ability to create a vivid and exciting lunar environment.
One of the best is 'The Star'. It's a thought - provoking story that combines science and morality. It makes you think about the existence of God and the nature of the universe in the face of a scientific discovery.