Hardest sound to pronounce-independent ofthe motherlanguage?

the ř in Czech can take a lot of practice

Arabic /q/ is a pain. Just try to say /iqi/ without having it come out as /iʔqɑi/.
Danish pharyngealized vowels are also strange. http://www.youtube.com/watch?v=z8VziyktyS0

<<I found all Japanese sounds nice and straightforward. >>

even the Japanese u?? I always have to think about it. "ok, lips like an "ee", tongue like an "oo"

French "fruit".
Here we have a nearly impossible combination: Fr. 'r' followed by a front rounded 'u' used as a semi-consonant introducing the 'i'.
No non-native speaker will ever be able to pronounce it correctly. It takes more than one life to get full command of it.

If you keep pronouncing fruit repeatedly, you sound like a pig.

What about double consonants in Italian? I'm pretty certain I enunciate those as stops.

No man u are yanking my chain.
Double consonant in italian is very easy.
Ok,i admit,maybe not easy like A E I O U but in one day or two u get it.

Italian is maybe the easiest language to pronounce.
Even spanish is harder, th,j,ñ,ll,what more?
Note that im not spanish.

LL is no longer pronounced properly (/ʎ/). People pronounce LL like consonantic Y, so you can simply so the same. Anyways Italian does have the /ʎ/ sound. The same happens with ñ,in Italian gn has the same pronunciation than ñ. Th is like in English so if you know English in Spanish it's the same. J can be aspirated on the other hand like in Andalusian Spanish and American Spanish if you don't want to deal with /x/.
Spanish is objectively the easiest language to pronunce as you can see.