Practical tip: Search for 'Hercule Poirot reading order' and you'll find a dozen infographics and listicles. Print one out, stick it on your fridge, tick them off. It's satisfying. I used the list on the official Agatha Christie website. It's just a straightforward chronological list by publication year.
I'm a linear thinker, so this worked for me. I enjoyed seeing the changes in society seep into the backgrounds of the stories—the cars, the fashion, the attitudes. It turned my reading into a bit of a historical survey as well. Took me about two years of casual reading to get through them all, and ending with 'Curtain' was an experience I wouldn't trade for a haphazard approach.