Out of all the languages spoken around the world, choosing the easiest one to learn is tough. There are many ways of deciding which one is the easiest to master. One (dumb and tiring) way is to try learning each one of them and then decide which one was the easiest. There are problems with this approach, unless you are a 'Braniac' and blessed with a very long life.
It's not possible to do so, as the number of official languages in the world is 161. However, you do not need to be a braniac to decide which one is the easiest language to master. All you need to do is narrow down using some simple criteria, which we will see ahead.
Another way is to conduct a nationwide or (better even) a world wide poll to answer this question. This would also take a lot of time. Instead, what one could do is, study what factors make a language easy to learn and subject the available list of language courses that you could enroll for, to those criteria.
Learning a foreign language is opening up a whole new treasure of arts and literature. It's your introduction to a new culture. Let us see which will be the deciding factors in determining the easiest language to master for you.
Your Native Language
This single factor will narrow down your choices a lot. What language you already know very well, will decide which you will easily master. With mastering of your mother tongue and your native language, you have acquired pronunciation skills of certain phonetics and mastered the ability to read at least a single script. The language you master with consummate ease would be the one which shares the maximum similarity with your native language, in terms of phonetics, script, and culture.
No languages of the modern world were developed in total isolation. Every language has borrowed something from at least one of the other languages. They were invented by people and as people spread, adapted, and settled in different regions, their languages changed too. All languages over the world share common origins and similarities with other languages. All of them can be grouped into about 8 to 10 major families. They share phonetic and word root lineage with each other. English language belongs to the Indo-European set of languages, which share a common lineage.
So, if English is your native language, then German, Spanish, French, Italian, and other languages of Celtic, Germanic, and Latin origin will be easiest for you to learn, as they are sister languages of English. They share a common origin and therefore, their scripts and phonetic pronunciations are similar. The roots of nouns and verbs have a common origin too, which will make grasping easier. This way the easiest European language to learn for native Americans is German, as it is a close cousin of English . The easiest Asian language to learn would be difficult to point out because of the large gap which separates the two cultures now, in terms of script and pronunciations.
Exposure to that Language
To learn a new language, one must hear it being spoken and get exposed to it. One must have somebody to talk to, in that language. This way, the learning becomes easier. Many foreign language teaching institutes bridge the gap of direct exposure to the language, with audios and videos from the culture of that language.
So, the easiest foreign language to learn would be the one which you can hear being spoken and slowly learn through direct interaction. You master the spoken form of the language first and then learn to read and write it. Personally, I feel that the process of every language you learn should mimic the way you learned your mother tongue.
This is the easiest way to learn a language. Of course, you should be able to find a teacher who can do that and teach you the language in that fashion. If you live in the US, the Latin American languages will be easiest to learn as you may have Latin American people around you and their culture in the form of movies, songs, and literature.
Time You Devote and Practice
This is perhaps the most important factor. Even if you have the advantage of familiarity with a native language and exposure, you may or may not find that language easy, as it is directly proportional to how much time and effort you put in to master it. There is no substitute for practicing speaking that language and working hard on the vocabulary. It takes a lot of repetitive effort till the words get registered in your mind. So, the easiest one to learn would be the one in which you put maximum practice and effort.
If the language you learn has a similar type of grammar, to your own native language, then mastering it is certainly simpler. Spanish language is found to be easier to learn for American and even European people. Then, there are some languages like Sanskrit, which have a different and highly refined grammar, but it might be the hardest to learn for you, as it is different from English in most respects. So, choose a language with simple grammar. This too depends on the amount of time you can devote to learning it.
Your Desire to Master That Language
Ultimately, what can make the language easier or easiest to learn is your intense desire to master it. Suppose you meet a girl from a foreign country and fall in love with her, but she speaks only the Japanese language. What do you do? I bet you will try to master Japanese with all your heart, just to reach out to her.
All these factors will decide the ease of language learning. There is no one language, that can be deemed to be easiest to master, other than sign language, the language of music, or as Harry Potter fans would agree, the easiest one is 'Troll Language'; all you have to do is point and grunt.