The original Foundation trilogy or, rather, sequence, since it's actually a set of short stories/novellas rather than actual books, is frequently cited in lists of "great SF".
It fully deserves its place. Although none of the stories in themselves are out-and-out classics, the cumulative effect is utterly amazing, recounting an almost-plausible massive future history all hung on the genius concept of "psychohistory", which is used as the theoretical filter through which the individual stories play out.