I know you said in the world but what about Klingon opera? 
My vote would go to modern faux country music. Singing pretend sad songs for the sole purpose of raping the audience just doesn’t do it for me. Plus, silly hats.
British Rap is pitiful ,many reasonably bright children would be ashamed to admit responsibility for the tripe they come up with.
I wouldn’t be surprised if they played it to the prisoners in Guantanomo bay in order to get confessions.
Any music turned up loud enough so that I have to listen to it. For example: that loud party in the cul-de-sac behind my house the other day, cars next to me at stop lights with their subwoofers cranked up, or a cow-orker playing adult contemporary/smooth jazz all day until I want to smash their radio.
It’s not that I don’t like loud music, I just don’t like other people’s loud music. When I want to crank it up, I use headphones or head out for a drive on a back road someplace.
While I’m not a fan of most rap or most country (although I love bluegrass, and it’s a fine, fine line sometimes) or most jazz, I think the *worst *music for me to listen to is some of the Middle Eastern and Asian music that uses a different scale than we do. Half the notes just sound flat or sharp to me, even though I know they’re perfectly in tune for their own format. Sends creepy crawly feelings up and down my spine.
Confirmed. Also;
Canto-pop.
I’ll go along with that. Sometimes I’m even offended by music that I actually like, just because some asshole neighbor is playing it so loud I’m forced to hear it in my own home.
Just about anything you’re forced to endure over and over again while working. At the top of my list is happy-happy retail Christmas music. Then, the Rat Pack*.
*Yes, yes, I know, everybody loves each and every one of those stylish fuckers, but I’d rather listen to an experimental 8-hour duet between a creaky door and Yoko Ono.
I’m going to have to go with Beijing Opera. I’m an American student living just outside of Beijing, and although I realize it’s culturally significant, etc., etc., every time I get into a taxi playing that warbling horror I seriously debate the pros and cons of just diving headlong into traffic to make it stop.
God, I studied in Israel for a year in college, and the girl who lived below me (paper-thin walls in the dorm) played what I expect was Egyptian pop music at high volume. It was like torture.
Some of the volunteers I knew in Bulgaria would put chalga (the Bulgarian version of Balkan turbopop - a blend of Turkish and Roma music with Western pop, with banal lyrics, and usually served with an incredibly trashy music video) on this list, but I kinda like it. Here are some examples.
My vote, though, goes for the crap techno music they play in most dance clubs. Hate it hate it hate it.
Any of that hip-hop-beat-with-Middle-Eastern/African/Asian-melodies that seems to be prevalent in Europe and fawned over by the anything-but-rock types.
Hell, I’d buy that! 
Now playing: John Zorn, Six Litanies for Heliogabalus
When I was in E.Jerusalem a few years ago there was an I.D.F. sentry post over a part of the market and very close to an Arab music stall.
To wind up the Israelis they played one song literally over and over again at full volume. Get this, just the ONE tune for hours on end.
The place I was staying at unfortunately was also close to the stall and people who couldn’t speak one word of the language became word perfect with its lyrics.
We truly hated that song and we also inspite of not being Israelis or even Jewish ourselves truly hated the stall owner,his family,his friends and even his donkey if he owned one.
Ah music,the international language!
Consider this; all over the world there are people who weren’t good enough to be Michael Bolton, Celine Dion, and Don Ho.
I mean, if you wanna get technical, William Hung is way worse than Rickey Martin, and there are people worse than William Hung. But let’s assume we’re only talking about genuinely professional music, in which case there are three I’d nominate:
1. Foreign music in a different scale. WhyNot already mentioned this, but a lot of music, such as Asian music, is on a completely different musical scale and so to my ears sounds like a crate of poorly tuned xylophones tumbling down a flight of stairs.
2. Terrible Musicals. While Andrew Llyod Webber is bad, I believe “Grease” is actually much worse. “Grease” is a disgrace; almost every song is horrible, and the only one that isn’t, “Beauty School Dropout,” is just forgettable.
I also think “Annie” is pretty bad, but it at least has one good song. Not “Tomorrow.”
3. Adult Contemporary Music, such as Celine Dion, Michael Bolton, Clay Aiken, etc. Frankly I don’t know which one of those is wore than the others. They’re all the same product, more or less.
Any post-Coltrane “doodly jazz”, where the original melody is vivisected, and all the players go off on their own solos - often all at the same time. Music played by musicians for the delectation of musician-wannabes. I love jazz music from back when it was essentially pop music. But it made a turn in a direction where it abandoned the majority of listeners to become a masturbatory exercise in display of “chops”. Nobody cares if you can use obscure chords if it doesn’t work in the context of a song.
gaffa: You mean you honestly can’t appreciate The Shape of Jazz to Come? Or On the Corner? Or Selection Sixteen? Anyway, you are factually incorrect: There are plenty of us who care about chops and complex music not in the service of a simplistic song.
I will bite on your troll-bait and scream “You are wrong, wrong, wrong!!!”
Sorry, I didn’t mean it as troll-bait. Just a specific low I could remember. If you disagree, that’s your privilege and right.
Just listen to it over there, please? 
Country music. All of it. Even the classic stuff I’m supposed to like, your Patsy Clines, Hank Williams Srs., etc., I find intolerable.* It actually produces a physical response in me, making me uneasy and agitated. If country music is playing, I have to get away as soon as I can.
I realize it probably sounds like I’m exaggerating, but I’m not.
*Johnny Cash excluded. He transcends everything.
I own a CD called Love, Peace & Poetry: Brazilian Psychodelic Music. It’s an interesting enough collection wuith some decent stuff ( if you can tolerate late 60’s/early 70’s style psychodelia ), some curiosities and one true horror. The song “Let’s Go” by The Sound Factory features what has to be the worst vocals ever committed to vinyl by a rock band. Which is saying something.