The books are generally better than the films as they allow the reader to imagine what everything and everybody looks like......the films rarely conform to the original book...except David Suchet as Hercule Poirot!
I like Peter Ustinov as Poirot in Death on the Nile and Evil under the Sun but my favourite has to be Albert Finney in Murder on the Orient Express. Excellent cast and very well played by all concerned. Dont bother watching the Kenneth Branagh version, its awful.
Moving away from Agatha Christie, the best film version of a book is, IMO, Frederick Forsythe's book 'The Day Of The Jackel'. I have not seen a film which is closer to the book than this.