Well, everything is in the title 
Which word(s) in your target language do you have the hardest time pronouncing?
In English I would say words like squirrel or jewellery, and in Chinese it is not really a word but rather sounds: j and q. No matter how hard I try they always sound the same!
What’s yours?
I totally agree with “jewellery”!! So hard to say…
About Chinese, I usually struggle with the ü
I’m copying this here from our Discord, someone said:
a squirrel whirled the earl’s jewellery around a rural brewery
That’s a lot for one sentence
It’s crisp in English for me. I think i only got the hang of it 2 years ago in my 10 years of learning this language. My tongue still feels awkward to pronounce sp sound at the end of the word 
In Chinese I have a lot: 解决、继续、自己、出租车、自行车等等😳
In Mandarin, the sounds “c,” “x,” and “s” are challenging for me. They are very similar, making it difficult to distinguish them. I tend to hear all of them pronounced as “x” in my native language. 
I think we had this topic before… sounds quite familiar. 
I keep pronouncing the schwa-sound like in 可乐 not schwa-y enough and more like a German short “e”, so people sometimes don’t understand me.
My husband’s name, Thuận, is very difficult for me to say correctly. 
Especially if I want to call loudly with the correct “heavy” tone to get his attention, I just can’t do it, LOL.
Both “th” sound and “dấu nặng” short low tone are very challenging for foreingers.