Most difficult to understand spoken language? in terms of listening comprehension.

[Dragon27]

Chinese, as far as I'm aware, is chock-full of homophones (even when you distinguish the tones). Wiki even has a dedicated page describing the phenomenon of homophonic puns in Chinese (which is telling). This overabundance of homophones is invariably mentioned as one of the reasons why characters is a good thing (they allow you to distinguish between homophones in writing). Although one doesn't have the benefit of seeing the characters in actual speech, so it's not that much of a relief.
It seems to me (intuitively) that a (relatively) greater number of homophones in a language should be compensated in some way, i.e. there should be more rigidity in the way you can combine words, or something of that sort.

[ninuno]

French has most of the ending consonants silent, so as i learn more by reading, it is very hard to relate the sounds to the words,

e.g. [o:] is really 'haut'

that goes the same with nearly so many words

[ninuno]

polish and maybe czech is harder to understand than russian ,

portuguese is harder to understand than spanish ,

korean has much more vowels and again the ending consonants as french has ,

moroccan arabic is hard to understand as many say .

[Tristano]

I studied (not necessarily learned) a multitude of languages. The one that really put me on my knees has been Dutch.
I remember after studying a few months and not being able to understand anything from Dutch podcast, I tried listening to a German podcast and despite never having studied German before I could understand quite a lot thanks to having studied Dutch. Dutch was still incomprehensible for me.

The other two languages that gave me troubles were English and in a lesser extent French.

There are two types of French speakers: the ones you can understand without troubles, and the ones that say "EOEUEUEUE!!!!" and when you ask them to write down what they said they produce a book of 1000+ pages.

English has a such insane spelling that it took me a lot of practice to get a gist how spoken English maps written English.

Spanish spoken at 1000 words per seconds give me no troubles at all on the other hand.

[sporedandroid]

I studied (not necessarily learned) a multitude of languages. The one that really put me on my knees has been Dutch.
I remember after studying a few months and not being able to understand anything from Dutch podcast, I tried listening to a German podcast and despite never having studied German before I could understand quite a lot thanks to having studied Dutch. Dutch was still incomprehensible for me.

I seem to have the same issue with Icelandic. I understand spoken German better than Icelandic despite studying it less.

[tussentaal]

I'm learning Norwegian, but seem to understand Swedish better because the Swedes enunciate better.