In your opinion, which language is the most difficult in the world?

I think it's my native language, Chinese.

@Free_Elo_ATM said ^

I think it's my native language, Chinese.

Chinese is indeed difficult, but in my opinion, the most difficult language is Russian

Japanese, 4 different words for the same.
A Korean once said:
When we learned English, we started crying, and we ended smiling.
When we learned Japanese, we started smiling and we ended crying.

The most difficult language to learn, i don't know, but the most difficult language to listen to in real life is when people talk like "Negative one" does here, but aren't putting it on for fun. :D
https://youtu.be/nXhZIyGTz5Q?is=x9NluaSFAkO4oRnd

Chinese.

1. Measure Word Storm: As a Sino-Tibetan language, Chinese has a rich set of measure words, so you can say "a cow" (一头牛) but not "a set of cows" (一套牛) or "a building of cows." (一栋牛)
2. Polyphonic Character Party: Check out this sentence yourself: "If a person is capable, doing one job well leads to doing another well, one after another" (人要是行,干一行行一行,一行行行行行).
3. One Word, Multiple Meanings: Take the word '意思' (meaning) for example; it has different meanings in different contexts. For instance, in '你这人真有意思' and '这个词的意思是“水”', the two '意思' have completely different meanings.
4. 的, 地, 得: All three are pronounced 'de,' but they function completely differently—'的' modifies nouns, '地' modifies verbs, and '得' indicates the degree of adjectives or adverbs. For example, '精彩的故事' (wonderful story), '高兴地跳了起来' (jumped up happily), '红得发紫' (so red it turned purple).
5. Homophones (extension of point 4): Go look at Zhao Yuanren's 'The Lion-Eating Poet in the Stone Den' (施氏食狮史) and 'Ji Ji Beats the Chicken' (季姬击鸡记) yourself; I won't explain.

Summary:
English: Cry at start, smile at end
Japanese: Smile at start, cry at end
Russian: Cry at start, cry at end
Arabic: Cry at start, cripped at end
Chinese: Smile at start, died at end

Here's another typical example:

In English, words for "death" include roughly these:

die、dead、death、decease、deceased、depart、departed、expire、perish、pass、demise、exit、fall、slaughter、execute

Whereas in Chinese, the terms for "die/death" are something like this:

死、死亡、去世、逝世、走了、没了、不在了、过世、离世、故去、离开、亡故、身故、长眠、安息、与世长辞、寿终正寝、仙逝、作古、永别、谢世、辞世、长逝、咽气、断气、蹬腿儿、翘辫子、归西、上西天、见阎王、挂了、凉了、嘎了、牺牲、捐躯、殉职、遇难、罹难、夭折、暴毙、横死、自尽、轻生、殉情、殉难、阵亡、就义、成仁

@VigourGirl said ^

I think it's my native language, Chinese.

Chinese is indeed difficult, but in my opinion, the most difficult language is Russian

Ik russian but u can be true i kinda forget after yapping in english too much.ти русский? Или не?

I assume by "most difficult", you mean "most difficult to learn for English speakers"? Because I think in that case, you'd be correct. Chinese, Japanese, Russian, Arabic, and Hungarian are the big monsters put up at the top of hardest language lists. But learning Russian, for example, would be 10x easier to learn if your first language was Polish or Bulgarian or something in the same family. The "difficulty" really lies in the degree to which it's different from your native language.

@Free_Elo_ATM
I don't think word lists are a good comparison for language complexity. You could make a similar list of "dying" words in Russian. The language allows for the free construction of all sorts of extremely specific words.
умереть, умерший, умерших, умершим, умирая, умирающий, умирали..... итд
and these are ONLY the words built from the VERB from the root "мерт-", there are a few more of course

I can't say much about the other languages, but as I've studied Russian, I can give you an idea of how much of a pain it is sometimes.
Russian uses 3 genders, and usually they're very easy to tell apart by the final letter:
-а, -я: feminine. -о, -е: neuter. Ends in a consonant: masculine.
There are exceptions, of course: Дедушка(grandfather), some old Slavic root words ending in -мя are neuter: время(time), пламя(flame).
The real headache is when words end in a softened consonant using -ь. These words are unpredictable and you have to memorize all of them 1-by-1. Любовь(love) feminine, but Камень(stone) - masculine

Why this is a pain is because of the case system. Every Russian noun and adjective has 6 different forms, and they MUST agree with each other in gender:
чёрный кот (black cat)
чёрная собака (black dog)

and in case ("genitive case" in this example):
чёрного кота (of the black cat)
чёрной собаки (of the black dog)

But notice that the forms are DIFFERENT depending on the gender of the noun. They're different if it's something masculine or feminine. There are entire tables you have to memorize just to be able to put words in the correct form:

This is further confounded by Russian prepositions. Look at these two: в (inside), на (on, on top of). Every noun in Russian will "more naturally" prefer one or the other. It's like in English, we say "I'm in the house" but NOT "I'm in work". We say "I'm at work". Usually they make sense, more or less.
I want to say "I am in the grocery store". You have to know the word "grocery store" is магазин. This word takes в (inside of), which is logical. The word is masculine, so by our memorized tables, we know the proper form for IN the store is в магазине.
Я в магазине.
But this happened to me recently: I wanted to tell my friend "I am at such a level in Russian where I need to...". And I realized, how do I say "I'm at such a level"?. The word for level is уровень. Notice it ends in "-ь", so I have to memorize whether it's masculine or feminine.
But I didn't know.
I also didn't know whether уровень takes В (inside) or На (on). So I was placed in a conundrum where any of the following could have been correct in my eyes:
я в такой уровни, что... (feminine, takes В)
я на такой уровни, что... (feminine, takes На)
я в таком уровне, что... (masculine, takes В)
я на таком уровне, что... (masculine, takes На)
but only one is actually correct. And there is literally no way to know. Dictionaries will tell you the gender of the noun (in this case it's masculine) but they don't tell you whether it's В/На.
And this is just talking about nouns and adjectives. Not even about HOW the cases operate, which case you need, how the verbs conjugate and require certain cases or constructions.