I have been thinking on this answer and I agree with Jeza.
Took me back to Parkinson's shows on TV and thinking about the interviews I really enjoyed. Those that come to mind were either spell-binding, hugely entertaining or enjoyable for their candidness - a quality evermore infrequent.
In no particular order here are some of my favourites:
Commissioner Catherine Bramwell-Booth, Peter Ustinov, Dame Edith Evans, David Attenborough, Kenneth Williams, Anthony Hopkins, Elton John.