What is the most difficult language for a native English speaker to learn?

In The Economist, a highly interesting article raises this question and discusses the problems various languages pose for anglophones. For example, as illustrated in Mark Twain's humorous quote below, English speakers often struggle with gendered languages:

"German has three genders, seemingly so random that Mark Twain wondered why “a young lady has no sex, but a turnip has”. (Mädchenis neuter, whereas Steckrübe is feminine.)"

Of course, Twain was joking and the assignment of gender to nouns in languages such as German has nothing to do with the noun having feminine or masculine characteristics.

The article mentions that the further a language is in terms of its linguistic relation to English, the harder it will generally be for an English speaker to pick it up. It seems intuitive that for a Brit, learning Mandarin would be a greater challenge than learning German.