I don't quite say she is my favorite mystery writer, yet in many ways she must be. I have all her mystery books (she wrote a few other things under a pen name,) have seen most of the adaptations of them, and have read several biographies. I am always impressed at how her resolutions are rooted in a possible reality; there's no fudging, no deus ex machina, and the clues always turn out to have been there all along. She must have had an interesting mind.
But her characters, they're not my favorite people, most of them, and that is why when I am feeling sentimental about mystery stories, I'm more likely to pick up a book by Dorothy L Sayers or Rex Stout, maybe a few others.
I would be willing toabsolutely faire l'amour with Lord Peter Wimsey or Archie Goodwin. Hercule Poirot, not so much. There are lots of sympathetic characters in Agatha Christie's books, though, or else people wouldn't keep rereading them. However good the story may be, it's the characters that draw you back to it again. When I was younger, I didn't enjoy the Miss Marple stories as much as the Poirot ones, and largely ignored the books without either major character. Now I find some of those, such as Towards Zero, are among my favorites. They have a gripping depth to them that I can appreciate as an adult.
I did enjoy the Tommy and Tuppence stories all along; they were more familiar to me in terms of character development. But they are unevenly written, some very good, some less so. These you read for the characters; less so for impressive development and denouement.
Here's a nice brief biography of Agatha Christie, with a short examination of her writing and a good simple bibliography.
The original Bedside, Bathtub & Armchair Companion to Agatha Christie is a favorite book of mine to page through now and then; you can find used copies of it pretty easily: this is an updated version. There are other more complete companions, this one's just kind of fun and different.