For me, The Murder of Roger Ackroyd is the best Agatha Christie. Go in blind. For non-Poirot books, Crooked House and Murder at the Vicarage are also top shelf.

If you like Christie, I would also recommend Louise Penny. She's very stylistically and thematically similar. Still Life is a great mystery and also a nice window into a cute pastoral Canadian town.

For something off the wall, Leech by Hiron Ennis. A detective is dispatched to a snowbound manor house to investigate the death of his predecessor. However, the detective is a sentient parasitic leech hivemind and the killer he pursues is an alien fungus body snatcher.