Actually, I think my all time favorite mystery author is Dorothy L. Sayers and her Lord Peter Wimsey mysteries.
But there are a lot of great series which either take place in an interesting location/local customs, etc. or have interesting characters (or who cook and include recipes, etc.) Such as Arthur Upfield's half Australian Aborigine detective inspector Napoleon Bonaparte, or the Amelia Peabody Egyptian archaeologist c. 1900 mysteries, or Brother Caedfel medieval mysteries, etc.
I never cared for Agatha Christie -- the plot was always the same and you never learned anything from the books.