The Murder of Roger Ackroyd
It was, at the end of the day, an Agatha Christie, but I went in thinking "boy I hope this isn't just straight-up worse than the BBC adaptations" and it wasn't! I don't think there's a way to neatly translate this to the screen without losing the gut-punch of the twist (which I managed to call early on, but more out of a familiarity with Poirot than out of obviousness), and it was indeed terrifically executed. I know the central conceit is a bit hackneyed now, but it felt well done and earned.