some of the known hardest languages are - Chinese; Arabic; Japanese; Hungarian; Korean; Finnish; Polish...but i feel it all depends on your mother tongue and/or probably any other language you know apart from your mother tongue, that how quickly one learn new language.
There is a Hungarian tongue twister that is a good example of why Hungarian is on this list: "Te tetted e tettetett tettet, te tettetett tettek tettese" But this is not a representative example of what Hungarian is like.
Btw there's a saying that Hungarian originated from a Slovakian guy falling down the stairs and cursing.
Czech has different specialties, like vowelless words (vchrstls) and vowelless tongue twisters (strč prst skrz krk).
Also the weird-looking "nenaolejuje-li mě Julie, naolejuji já ji", which is a surprisingly normal sentence (unless Julia lubricates me, I will lubricate her).