Honestly, I feel like the whole 'best-selling' label for Christie is a bit misleading if you're trying to judge her actual work. 'And Then There Were None' sells the most, obviously, and it's a masterpiece of locked-room tension. But saying it's her 'best' novel? That's a different question. A lot of readers come for Poirot's mustache-twirling genius in 'The Murder of Roger Ackroyd,' which pulls off a trick so audacious it's arguably more influential on the genre itself. Or the cozy, English village perfection of 'Murder at the Vicarage' with Miss Marple, which offers a completely different flavor of murder.
My point is, comparing her best-seller to her other novels is less about quality and more about what kind of Christie experience you're after. 'And Then There Were None' is pure, relentless suspense with no series detective. It's a standalone engine of paranoia. Her other books often have the comfort of recurring characters and a puzzle you can try to solve alongside them. The sales numbers just tell you which entry point is most popular, not which one is the most clever or the most comforting on a rainy day. I actually think some of her less-hyped post-war stuff has a darker, more interesting psychological edge.