You use the phrase 'level of achievement'. 'In music', one either writes (composes) music, or performs it.
I would have thought that the 'highest achievement' would either to have composed a piece that most people get the most out of, or to perform a piece that the performer gets the most out of, or that the audience gets the most out of. It all sounds very theoretical, doesn't it? I'd say Beethoven was a high 'achiever' in a sense, but (fortunately), its subjective. Horowitz (at his best) and Heifitz I would rate as 'high achievers' on the performing side. But who is higher, leave alone highest, I leave to people who must think they have better things to do than write or perform music.