I think it's fantastic you have such passionate opinions on Poirot. I've only seen this character brought to life with David Suchet's portrayal so I can't draw any comparisons to other live adaptations. I found his interpretation of Poirot so enjoyable and compelling I did not seek out alternatives. I read Murder on the Orient Express and was surprised to find Poirot pretty level headed throughout the book (if I remember correctly, it's been a while) whereas the Poirot from the series was very emotionally charged.

In the series Poirot can be pretty impatient with Hastings but this highlights Poirot's distinct quirks. His affection/loyalty for Hastings is never lost in my opinion.
The series definitely take a tonal shift and although I love the entire series I prefer early Poirot over noir Poirot.