[knivesout]

I'd say short stories. Some of his novels are very good indeed - Childhood's End or Rendezvous with Rama for instance - but for the most part, his brand of classic SF works best in the shorter format, where the masterful playing out of a kernel 'what if' can exist more or less on its own without needing further layers of human interest and so on. Having said that, he is a very humane writer, even if characterisation is not his strong suit.