It also depends if you are talking about SKILL too... a lot of artists on here write stuff, but can't read sheet music or play an instrument or haven't gone to any school for music, but they are still artists, and some of them very good.
You can also judge based on complexity and how carefully they craft their work... there are people who pay close attention to every single note in their piece, spending hours making sure everything sounds just right, and others who just toss cliche chord progressions in and either write a melody or record vocals on top. Some people use complex theory to shape their pieces in certain ways, while others simply rely on their ear.
Alternately, you could judge people on how successful they are. This is tricky, as exposure is a difficult thing to measure in a place where it comes to a few instantly, others rarely, and some never at all. Just like in the mainstream music world, not all the pop stars are REALLY the most capable musicians or composers, they just happened to have been in the right place in the right time and done the right thing.
No matter how you approach this issue, I don't think there's ever a clear answer. Certainly you can make a list of your favorite artists in each category- for instance, in classical, you could rattle off a good dozen names of people who post works under classical who are generally considered to be (or were) the higher end of it... Bosa, Edgen, Deadlyfishes, BrokenDeck when his keyboard is not trying to hinder his piano mastery, etc. However, I would have trouble picking among all their number one person who, even in just that section, is the best. They all have their own uniqueness to them that I would never be able to decide one over the other. Perhaps if you said... "who is the greatest [dark orchestral] music artist of NG?" it would be easier to answer, when the individual niches and characteristics of each composer comes into focus.