Murder on the Orient Express
Agatha Christie, Murder on the Orient Express.
The weirdest thing about this story is that I kinda knew the twist, simply from cultural impact it had; but I still managed to be surprised when it was revealed. You see, I avoided confirming to not spoil the surprise and this was a good choice. The story is very carefully build up with details revealed in exactly such a way as to confuse the reader just enough. So it really felt like a mystery.
Note: I listened to the audiobook narrated by Kenneth Branagh.