Just some quick comments:
1) It is obvious you can't tell which language is the most difficult to learn, because it's a subjective thing. "Difficult" means
you are not able to do it well, but someone else might have no problems. Playing the guitar might be difficult for those who never learned how to use one, but it's pretty easy and enjoyable for many others. So I think I wouldn't have problems with Spanish grammar, since I'm Italian, but I doubt I would find Chinese ideograms quite easy.
2) Every language has some aspects you would find difficult, and others you would not find so difficult after all. All this is still very subjective. Some aspects you might want to consider are grammar, pronunciation, writing, speech registers, cultural aspects, regional aspects, etc.
3) The more a language it taught, the simpler it is to find what you need (more material = a better perspective and understanding). So English and Spanish, no matter how difficult and who finds them difficult, have a huge advantage: they are taught a lot. On the net you'll find an avalanche of websites to learn them, for free. Lots of teachers, lots of courses, lots of material. But now try to learn Finnish for example... how many forums to learn Finnish for free from volunteers are there online? As many as those for ESL? Hmm, no way. So less material --> higher probability of confusion and slower improvement.
4) What do you mean by learning a language? Learning the basic words? The basic grammar? Advanced vocabulary? Good style too? Perfect pronunciation? Learning about the major dialects too? And about the cultural aspects?
So the point is, at what level are you going to use a language you are learning? Maybe it's simple to learn how to say the most basic things ("hello, how are you?") in every language, but how about talking about eye surgery like a professor would? Attending a history lecture and understanding basically everything? You need "mastery" for that, and it's probably extremely hard to achieve for every language, no matter how subjective the difficulty might be.
Just my thoughts