Instrumentalists imitate the human voice more often than human singers imitate instruments. Evidence for the rule includes Louis Armstrong’s duets with Ella, Bach’s obbligato parts to sacred arias (I am thinking especially of oboe and cello), some styles of African drumming, and even some didgeridoo music. In the category of interesting exceptions–there are few–I’d put Bobby McFerrin (e.g., his duets with Chick Corea).
I suppose the most beautiful instrument must be the one that most closely resembles the human voice, man being the measure of all things and all that. If a mellow baritone voice (my personal favorite) were the goal, I’d nominate the French horn as its, um, avatar. Surely composers would have used the horn more often a century ago had they had today’s range of choices for mixing and matching dynamics. French horns simply have a lot of power, evidence I guess of their history in hunting, where it was important to be heard at a great distance. They have a lot of beauty, too: my ear always melts at certain passages in the Romantic repertoire where the horn soars wistfully above the fracas. But I wish I heard them more often in tandem with singers.