• Let's begin at the beginning: Wilkie Collins' The Moonstone is one of the earliest books that could be considered a detective novel, and still holds up well today. Of the adaptations I've seen, I prefer the 1996 version with Keeley Hawes and Greg Wise.
• The Wimsey books by Dorothy L. Sayers. The TV adaptations of Strong Poison and Have His Carcase with Edward Petherbridge are very faithful, as are the audio adaptations with Ian Carmichael.
• The Father Brown stories by G. K. Chesterton.
• Agatha Christie's Miss Marple mysteries, either as books or as the Joan Hickson television adaptations (I did a rewatch of them on this blog, 9 years ago by now).
• John Dickson Carr (and his less-than-opaque alias of Carter Dickson) has some good ones (including a few historical mysteries, and a couple that use time travel).
• For science-fictional mysteries, Isaac Asimov's Elijah Baley stories The Caves of Steel and The Naked Sun. And Alfred Bester's The Demolished Man.

I've also been exploring the British Library's Crime Classics collection, as a way of discovering some lesser-known 1930s mysteries. I've definitely found some good ones in there.