I thought Albert Finney in "Murder on the Orient Express" was the perfect Poirot. It may have looked like caricature, but Poirot was a caricature anyway (and I bet he had a whale of a time doing it). I was most disappointed when he was replaced by Peter Ustinov for the follow-ups: Ustinov never plays anyone other than Ustinov.

I agree with Scribbler about Roy Marsden and Dalgleish, though my memories of the latter are getting dim - I stopped reading P D James when it became apparent that she was suffering from the delusion that she was Dickens (not long after "Death of an Expert Witness", I think).