Georgette Heyer doesn't quite rise to the level of "favorite" mystery author, but after reading three of her mysteries, I can say that she's definitely on my reliable list.
Heyer is famous for her Regency romances, of course, but the mysteries were contemporary to the time they were written (mostly 1930s and early 40's, but there were a couple that cropped up a decade later in the 50s.) They're all "manor house" mysteries -- puzzlers taking place among England's idle, or semi-idle class. Very much the kind of scenario you imagine playing Clue -- was it Colonel Mustard in the Conservatory with the Candlestick?
The biggest problem I have with her writing is that she introduces us to a huge crowd of characters with similar names right off the bat. She does a good job of differentiating their _characters_ (which are usually a bit caricatured and flat, but still entertaining) but a few pages later you can't remember whether Figton Newby is the lawyer who is in love with Heather Mushingham, or if that's Stanton Biggby who's in love with Saffron Billingham. (And does "Miss Mushingham" refer to Heather, or is that her aunt?)
This sorts itself out after a couple of chapters, but I now don't start a Heyer mystery without a notepad.
Behold, Here's Poison is my favorite so far. It didn't start out that way: the characters are not very likeable, or at least they seem like they aren't. Most of them were under the domineering thumb of the victim, and they're kind of whiney and defensive, and prone to squabble. It perks up as soon as the chief antagonist shows up -- a cousin who is now "head of the family" and who is described as an amiable snake. Think Oscar Wilde as a master villain. He benefits most from the crime, he has a perfect alibi, and everybody, including the police would like to nail him.
But he also has this oddly beneficent effect on the story in that he, temporarily at least, often unites the bickering parties. The story itself plays on a hidden theme of amiability, and social poison, and all that.
The puzzle is fun, though the ultimate solution isn't memorable, the revealing of the situation surrounding it, and the little solutions along the way, make it worth it. The police characters are not so memorable, but they grow on you, and the story perks up particularly well when in Inspector Hannasyde's point of view.
Because the cluing is kind of interesting, I might do a little discussion of the big and small secrets in the story on the Spoilers blog. (I'll post a link here if I do.)
See you in the funny papers.