"Merlin's beard!" - Newt Scamander
It's a simple phrase, but damn is it effective at conveying surprise. Hopefully we get to hear some of the coarser variants in the sequel, such as "Merlin's most baggy Y-fronts".
Commonly found reading, sitting firmly in a seat at the cinema (bottle of water and a Freddo bar, please) or listening to the Mountain Goats.