i think, imho, Finnish is the most difficult language after Japanese...i sometimes tried to understand the way such cases as "essive" "ellative" "allative" etc. have to be used.....but is abominiously difficult to learn all of them....and then the fact finnish has (as in every ugrofinnic language) the infamous "wovel harmony", not to mentition the "consonantic weakening", (just squicking)...and then things like using a kind of "tonal pronunciation" which makes changin' the meaning of a word just by slightly swithcing its pronounciation (as in chinese)...definitely not the funniest language to learn....

one good note about it is, by the way, that Finnish has a very soundful and somewhat "spanish-like" sound that's very similar at the Italian's one (my mother language)...at least this....

i tried German, btw...and i must say that those 6 months spent at university (i'm 23) learning it helped me alot in understanding more than ever about English and other Anglo-saxon-germanic-heritage languages....but then i left it because was too hard learning how to tell the hour!!! and because ehr....my stereotypic thoughts about German were a bit too much toony

another very scruffy language but more soundful and french/english-like than german is Dutch....it has a lot of things borrowed (forever) from french and english and both in writing and sounding is "meltingly rough" or better "clumsly harmonic", and grammar is quite easy...way easier than german one for sure...its most exciting (not exactly the right word, but...)feature is that it has 5 or 6 way to say the pronoun "you" , the english like pronunciation of the "R" and "OE" diphtong pronounced as in "shoes"

then another language that drives me crazy (literally) is Hungarian!!! i soooo like it!! although it has some spooky finnish-borrowed feature (like wovel harmony, or case-like suffixes...for example "Magyarorszagban"..."to Hungary", or things like objective and subjective conjugation...which is almost a mistery again for me) it has a really exciting (this time is the right word!) pronounciations thats very different from other ones...for example "ZS" is pronounced as in "juillet" in french....or "dzs" is like in "jack" or even SZ that unlike the other languages, is not as in "ship" but as in "sick" and "C" is like "take" with a very strong "t":zake

but the number one is my favourite language forever: Portuguese (portugal's one, of course) the most roughly joyful, crustly, freanzy, overtightened language i ever heard!! with a really complex grammar but it does really worth an effort to study it!!
and so useful indeed!!! i'm improvin' my knowledge of Spanish of North Italy dialects, of French grammar and pronounciation , adn even British English (and somewhat also German...i'm not joking'!) just learning Portuguese!!! it's just amazing!!!