I'd go with Mecca whenever most people would be there during their pilgrimage. It might seem islamophobic but really it's just about the practicality of reducing an amount of the most religious people from the world's most violent ideology. Also it would be fun to see muslims spinning around on their prayer mats, when they can no longer locate Mecca.
Same for RIyadh. I can't think of many other major Islamic settlements that aren't mentioned, wouldn't get damaged in the chaos of the nukes, and haven't been destroyed already.