I feel Japanese is the easiest to pronounce for a native English speaker because all the sounds in the Japanese language are a subset of the English sounds (but NOT vice-versa).

Whereas there are several sounds in the Russian language that are not in the English langauge. For exmple "ы", "х", "ь", "ъ" and consonant clusters like "здр", "дв", "гд" and I am sure there are many more.