Most fans would probably say 'Foundation'. It has been highly influential in the science - fiction genre. Its scope is vast, covering millennia of future history. The concept of using mathematics to predict the future on a large scale in the form of psychohistory is just mind - blowing.
Some fans might argue that 'I, Robot' is the best. After all, it has been a staple in science - fiction for decades. It makes you think about what it means to be human in a world where robots are becoming more and more advanced. The different stories in the collection each offer unique perspectives on the relationship between man and machine.
However, there are also fans who believe that 'The End of Eternity' is the top Asimov novel. It deals with time travel and the implications of changing the past to shape the future. The way Asimov plays with the idea of causality and the moral dilemmas that come with the power to manipulate time is really fascinating.