Dr.Sheppard, you're right that Christie's name isn't as prominently mentioned as one would think it would be. Though Christie is mentioned lots of times in the various interviews and news articles, her name isn't on the teaser poster, although there is a "Based on the novel by Agatha Christie" line amongst the general credits of the teaser trailer. I'm not sure why "Agatha Christie" isn't featured more prominently.
I think that every writer who has contributed more than two episodes to the Poirot series has ups and downs, and many of them have made questionable adaptation decisions, but due to his pivotal work in defining the series and characterization, I believe I will give the nod to Exton.
I notice that some authors focus mainly on plot. One thing I like about Elyot's work is that he puts a bit more stress on characterization, and gives actors a chance to show off their skills (especially in Five Little Pigs).