Languages are generally rated in catagories for difficulty. English and Mandrin Chinese are Catagory 5 (the highest.) Russian is Cat4, German is a 3+(usually rated as a 4 now.) English, Russian and German are the Languages I know. IMHO, English has the largest deviation in rules and structure, Russian has the most difficult grammatical conventions and German has the hardest vocabulary (try the official word for speed limit in German...) I don't consider English the hardest, though. Because of the incredably large acceptable rules, you can say nearly anything and be understood
