I just came across an online list of the supposedly hardest languages to learn, and discovered I'd studied the top five, much or little, and number six (Vietnamese) was on my list.
Of course no one can make an accurate list, since there are thousands of languages, and you would have to go pretty deep even to decide which is hardest. None of the hundreds of languages from New Guinea or Africa were included, probably because the person who compiled the list didn't know much about them. And some of the reasons the author gave were pretty trivial -- the fact that Russian uses western letters, but in a strange way, the fact that Gaelic has a different word order.
My own theory is that the spoken versions of all languages are about equally difficult objectively, and that kids learn those languages at about the same age, to a similar level of facility.
But from the perspective of someone speaking any given language, of course those languages closest to your own are easier to learn, and some languages have more objectively tricky sounds, tones, clicks or clacks. And written languages that use thousands of ideographs or characters are of course also objectively tough sledding.
I am tempted to add languages whose adverbs, nouns, and even verbs shift with the relationship with your conversation partner.