What is the most difficult word in English?

It's hard to say. Chemical compounds can be very hard to pronounce, and many of the species Latin names and classification can be. Two that come to my mind are: pteradactyl and apteryx (both dinosaurs). Another word that comes to mind is tsunami (the tidal wave after an earthquake). Another is the word loch (a sort of small lake). What all of those have in common are unusual sounds that are not found in English as a whole. The ts in tsunami, the ch in loch, and the pt found in the dinosaur names (but the ps sequence may be odd, it's much more common). Most of the difficulty in English is from its rich heritage of foreign words from all over the world. (For example, loch is Irish and tsunami is Japanese.)

Can you help me?