As ionutulescu said, it really depends on your native language.
The easiest language to learn is the one within the same family, and the next easiest would be the language family that is close proximity.
For example, if your native tongue is a romance language (French, Spanish, Italian, Portuguese, etc) then learning another language within this family is the easiest. After that, it would probably be English, then the Germanic/Nordic languages (German, Swedish, Finnish, etc.). English as you may know is a mix between the romance and Germanic languages. Then after that it would be the Slavic languages like Russian, Ukrainian, etc.
East Asian languages may be the most difficult for westerners to learn simply because it's so different and unfamiliar, but for an Asian, learning another language would likely be easier than say, learning German.
Mandarin Chinese is a mixed bag -- in terms of speaking, it's surprisingly easy once you get the four tones; writing/reading is notoriously difficult because it's simply rote memorization, and is one of the few languages left that doesn't use an alphabet.
Learning to speak Mandarin is easier than most people think because Chinese probably has the simplest grammar:
No tenses: "I go yesterday", "I go right now" "I go tomorrow".
No verb conjugation: I be, you be, "it" be
No male/female pronouns: just a generic "it" (no he, she, they, we -- just "I/we" "you" and "it")
No plural: 1 apple, 2 apple, 3 apple
No complex adverbs: "It play piano so beautiful" instead of "she plays piano beautifully"
So grammatically, it's very simple: just nouns, infinitive verbs, adjectives, and 3 pronouns ("I/we" "you" and "it").
The hardest part is mastering the "tones" which is an alien concept to western languages. In Mandarin, there are 4 tones (i.e. a "word" can be said 4 different ways, like singing a different note):
Ma(1): mother
Ma(2): leprosy
Ma(3): horse
Ma(4): to scold; anger
So if you're talking to your Chinese mother-in-law, you want to make sure you're calling her "Ma(1)" -- and not calling her a leper, a horse, or scolding her.