I suppose which is worse depends on what you're looking for. Obviously, there is no real satisfying conclusion to Postern, but there is something whimsical about it, with Tuppence reliving her childhood, which makes it better than Elephants Can Remember, in my opinion. That book does have a conclusion, but it's rather obvious and the journey to get there involves slogging through countless pages of really dense and boring writing.
Then coming up on the inside rail is By The Pricking of My Thumbs, which is also pretty bad.
Yes, these were right at the end of her career. There's a sharp dropoff in quality with the last three or four - not including Curtain and Sleeping Murder, which were published last but written decades earlier.
Then coming up on the inside rail is By The Pricking of My Thumbs, which is also pretty bad.