Having covered movies, video games, anime, manga, visual novels and books, it's reasonable to finish off this list with a discussion of music.
Music is probably the best entertainment on Earth. Without requiring your attention or any effort on your part, it can deliver pleasure to your brain 24 hours a day, even while sleeping. This miraculous power to continuously present to the ears a pleasing mix of melody and rhythm that never gets old because there's seemingly always another way a song can go from before and yet still sound harmonious is as close to Heaven as humans may ever get. It's no wonder that the Christian description of paradise involved harps and singing, music seems to be the only pleasure on Earth that's completely pure (not mixed in with Earthly, bodily needs). In a way, music is the proof of the soul. If we were just machines focused on reproducing ourselves, neither making nor listening to music would serve any use. The fact that we love listening to music is proof that at least part of us has free will -- we are an existence separate from the rest of the biome which just goes about all its preset programs automatically. Whether an insect is any more alive than an insect simulation on a piece of software is really just a matter of interpretation -- but humans are definitely alive, because music definitely wasn't part of Nature's original plan for our existence.
If you follow a few simple rules, music will always sound good to the human ear. Sort of like how the human eye appreciates the beauty of symmetry, the human ear appreciates tonal harmony. It expects certain notes to come after other notes, and is satisfied when they arrive, all according to various mathematical rules involving the frequency of sound waves. But this explanation is somewhat deceiving. It's simple to say why and how music should be good, but it's extremely difficult to actually make good music. Just like it's simple to explain the rules of Go to a person, who only needs to place a stone -- that works like every other stone on the board -- once per turn on a board -- where every spot on the board is just like every other spot. It's probably the simplest game imaginable, and yet no human or computer has ever mastered the game even once in history. We are still getting better at Go to this day, the formula still hasn't been cracked, it's just that deep a game.
Still, the rankest beginner at Go can go to a Go institute and learn the basic rules of good play from the pros. That beginner will be a better Go player, at the end of his training, than any Go player who lived before 1900, no matter how ingenious they were. This means that having a grasp of basic principles really has elevated the quality of the game, and the same is true of music. Music in the past was just laughable, lacking chords, a beat, or really anything we think of today as music. All of it can be dismissed out of hand. Which means competitive music only arrived in the 1700's with our classical composers. Whatever the best music is, it was definitely made after this point, when we had mastered all the basics of good music composition.
The problem is, after this point, everyone insists that 'their' generation of music was the best, and no one can come to a satisfactory judgement. Unlike Go, music isn't a sport where you can decisively determine the winner and the loser. Most of musical enjoyment is subjective and has to do with the emotions it stirs within a listener. If people associate good memories with the music that was playing at the time, there's no way any other song can possibly compete with that. Since technically speaking all music is at an extremely high level, with no fault that can be found in the choice of notes, there's really just nothing left to judge music by. It's obvious that some songs are better than others, but it isn't obvious how or why this superiority came about, nor is it obvious which is better when both songs are beloved by rival factions and neither is willing to cede supremacy of place. The default position is that classical music is better than all current music simply because it is old and famous, but does anyone think that the classical music would have caught on if it were released today? Without any vocalists, without a guitar, drums, a synthesizer, or a bass, would anyone really like listening to this music if they hadn't been taught that they're supposed to adore it?
I'm a big fan of classical music, but aside from a few standout songs, classical music is 'elevator music,' endless strings of notes with no clear direction or purpose that just waste our time and at best allow us to easily fall asleep from boredom. To claim that this is the apex of music is just ridiculous. Beethoven's 7th symphony may be the apex of music. Beethoven's moonlight sonata may be the apex of music. But elevator music is not the apex of music. Therefore anyone who just reflexively says classical is the best, and the most famous classical musicians made all the best music, is just appealing to authority and can't seriously believe what he is saying if he would just stop and listen to most of the crap Mozart composed. ((And there's hundreds of hours of it, I assure you, I've listened.))
If we don't put classical music on top, the situation becomes even more chaotic. People who listen to alternative have zero interest in country, blues, jazz, swing, ska, punk, rap, soul, etc, etc. Everyone has their own favored genre and the line is very rarely crossed between types. A lot of music appreciation is just what you're exposed to as a kid. Your ears become trained to listen for the beauty of that particular type of music, just like ears become attuned to listen to the particular sounds your language makes and cannot distinguish the sounds of other languages. In addition there's the nostalgia factor, and it all adds up to music just being pure chaos.
Now, if it's a particularly good song, I'm willing to listen to it from any genre. But inevitably I'll retreat back to my comfort zone right afterwards and listen to 1,000 times as much music from my preferred genre, jpop. So it's kind of useless to say you're a fan of all music. If you're 99% a fan of one genre and 1% a fan of the rest, you're not a fan of all music. A lot of your preference for a genre is due to your emotional state and personality, the music that appeals most to you is due to you and the singer being on the same wavelength, so like magnets the listener and producer attract each other. Straying into another musical genre where your heart doesn't reside is like taking a tourist trip to Europe. Sure, you wanted to see the famous landmarks, but you never once wanted to live there.
So for music alone, I don't think there can possibly be an objective standard of quality. This one really is a choice that most personifies a human's free will. Nothing is determined in advance, and all of our preferences are unique to ourselves alone. As such, I'll just list my own favorite music and call it a day:
1. Nobuo Uematsu
2. Yasunori Mitsuda
3. Yuki Kajiura
5. Jun Maeda
6. John Williams
8. Hitoshi Sakimoto
9. Loreena McKennit
10. Sarah Brightman
13. The Cranberries
18. Nana Mizuki
20. Mami Kawada
21. Maaya Sakomoto
25. Hagita Mitsuo
There are three factors that go into my judgement of musical quality -- How many good songs did the composer make, how many times can you listen to them repeatedly, and whether there was anything particularly unique about the song composition or singer's voice that isolates it from all possible competitors. People who make one shot wonders are great and all, but your entire musical career is worth more than just one good song. The people above are composers of tons of good songs, or singers of tons of good songs, one or two just doesn't cut it. All of the composers, obviously, are unique, because music composition is one of the most intellectually and creatively demanding jobs in history. You don't become a composer by being run of the mill. The singers have a harder job, but for them it's all about having a uniquely beautiful voice. Perhaps they were born that way, or maybe they trained for it somehow, but you can tell one of these singers from the other in milliseconds, almost the moment she opens her mouth. Singers like Bjork and Sarah Brightman are appreciated the entire world over for their unique voice, if it were easy to replicate them someone already would have by now.
If you don't have a standout voice you need a standout style. Azusa's quiet, soothing music is a perfect example of a unique style. AKB48 has mastered the energetic and cheerful tune. KOTOKO's music is a sort of surreal blend of futurism and optimism. Yozuca* is this sweet mix of love and melancholy that no one else can even attempt. Once you listen to these singers enough, it's easy to distinguish them one from the other. Even without a standout voice (like Angela or Sarah has), they've become standout artists for the great musical niches they've captured, the platonic ideals they've embodied with their singer's souls.
Naturally all 25 are a treasure trove, but your mileage may vary depending on your own tastes. Furthermore, just because I stopped at 25 musicians, that hardly means I don't listen to more. You just have to draw the line somewhere, and these are the best of the best. If you're curious about these artists, just google their discography, find a batch torrent, and enjoy. You'll find out soon enough why I, and much of the world, holds these singers/composers in such respect.