I don't really have exactly 1 map i'd say is impossible, there are several others out there but here is just a few of them which i consider impossible.
hmm.. for an unranked i'd say Dm Ashura - deltaMAX [Unexpected] (that thing gets pretty damn impossible near the end)
Ranked/Approved: Kokou no Sousei [Chaos] (Nuff said.)
Ranked/Approved : The Quick Brown Fox - The Big Black [WHO'S AFRAID OF THE BIG BLACK] (for any normal human being, that map just seems impossible to fc without half/fail/easy mods)
Ranked/Approved : Shotgun Symphony+ [Impossibly Intense] (difficult as hell)
Ranked/Approved : Can't Defeat Airman [Holy Shit! It's Airman!!] (imba jumps, just seems to be impossible to fc it properly..Cookiezi is an exception)

i tried that shotgun symphony...it ís not hard map,it was made bad,really bad....and because of it ,it is the shittest map i played ever...

just a little lemon tree

S it go I dare you do it.

Why u call Shotgun Symphony shitty

http://osu.ppy.sh/s/40499

[Fear] diff

yeah, shotgun symphony is sucky, and all in all, it's probably not as great as some other mapping pieces, and it is probably a pile of shit.

but it has a soul and a heart and awp putted lots of effort in it and really made it something special, though it's not a particuarly good map, it seems to be enjoyable, and compared to the soul- and heartless anime-ranking-factory-massproducts, it isn't a shitty map, but just an old classic with the old classy charm that old things tend to have.

plus, it's so friggin hard that most people aren't even able to enjoy the map because they can't pass the very beginning of the map without no fail/easy/half time. and the map still is the most important map for showing off that you are pro at this game, because still the guy who gets the first fc on shotgun symphony will ever be remembered by the osu-community.

You need to specify if the most hardest/impossible map could be (or not) FCable.

we are talking about ranked/approved maps.

http://osu.ppy.sh/s/34

something fcable for me might not be fcable for you iMercurial...and yes Tanzklaue you wrote it really nice
i think i agree with you ,especially your last sentence is true.

I'm surprised nobody mentioned Rekka Answer Insane yet: http://osu.ppy.sh/s/39953
Most of it isn't *too* hard to play, but if you go by to-FC, so far only 13 people in the world managed to record an FC run on it. Not as bad as sousei but fun too

http://osu.ppy.sh/s/27448

3.2% pass rate, for what it's worth