Some great answers already and while the question asks for the most unusual flower, that's practically impossible given the subjective nature of such musings. 40 of the worlds weirdest flowers is a great list from which to choose a possible fave. Notable exemplars include Rafflesia keithii aka the Corpse Flower. read more
Perhaps more appropriately called the starfish flower, the Star Flower is another carrion plant (a plant that mimics the smell of dead flesh). Sought by plant collectors and gardeners because of its unique, striking appearance and large fuzzy flowers, this is a plant that is best kept outside. read more