As I'm not a sound linguist I really don't know.
But I've read that Mandarian Chinese, Finnish and the African click languages are rather hard to learn. English is an intermediate language to learn--it is not easy! Indeed, given how well accepted English is as the international language of communication, very few non-native English-speaking persons speak it well. Out of all the large international languages, Spanish and Italian would probably be the easiest to learn; Italian words are easy to pronounce; Spanish words are easy to pronounce and spell, and furthermore, the language is totally structured--even the the supposedly daunting 15 verb tenses.
Désolé, mais le français est aussi plutôt facile de maîtriser. My best friend speaks a comprehensible German, and he's affirmed it's a beast--hardest large European language outside of Finnish, in his opinion.
But whom cares?
Just because a language is difficult doesn't mean it is useful. On the whole, the romance languages are fairly easy to learn only because the Latin (and to a lesser extent, Greek) grammarians were so skilled at their vocation.
Here comes the controversial part. In my impartial judgment, Englsih--particularly American English--is perhaps the best all-around large international language. The Germanic base of our language was flexible enough to incorporate, primarily through French, all the advantages of Latin vocabulary and grammar, without loosing its meaty yet concise Germanic foundation. O there are just so many lovely yet accurate one and two syllable words in our tongue as to make it a poets' paradise!
What's more, our language is the most copious in the world, with something insane like over 900,000 words choose from. (**Note** I deliberately put the prepositon 'from' at the end of the preceding sentence rather than directly after the function word 'which' as a quick (perhaps specious?) example of how English, where both Latin and our own 'Germanic' grammar are concerned, enjoys the best of both worlds **) To this very day native English-speaking people--again, particularly Americans--are great at pioneering new vocabulary. Perhaps this is an outcome of just how good we Americans have been at pioneering new ideas and technology in recent history. (Too bad we've turned right around and begun giving it all away to 'foreign competition' and illegal immigrants.
) Finally, the comparatively large degree of social freedom we Americans enjoy has rubbed off on our tongue. While I admit that, structurally, some of the romance languages (especially Spanish) are more flexible as it relates to subject, verb and object placement, we've nevertheless been able to surpass this advantage (which may actually be a disadvantage if it leads to grammatical arbitrariness) in other areas.
English must be one of the best overall languages in the world.
Keep up the good work,