One name only and state your reasons briefly.
If I had to stick to one director it would probably be David Lean.
Reasons: He had a long career(about 40 years) and made great films in several genres. In addition to his classic epics (Lawernce of Arabia, Dr Zhivago, Bridge on the River Kwai), you had his Dickens adaptations (Great Expectations, Oliver Twist), comedies (Blithe Spirit, Hobson’s choice), romantic drama(Brief Encounter) as well as In which we Serve which is hard to classify.
Maybe only Kurusawa has had a comparable career and I admit I haven’t seen too many of his films and was a little disappointed with the couple I have.
What’s your choice?