My own. :)
I actually do often audition equipment with horn music, sometimes with recordings of my orchestra.
But to give a more serious answer to your question, the human voice would be the one to use, and not just sung, but also spoken. The horn is not a bad second choice, though, as it has one of the more pure timbres, acoustically speaking, among the orchestral instruments.
Some folks recommend piano because it is a difficult timbre to duplicate. If you want to try another very complex tone acoustically, than the oboe is a good choice. But if the more pure timbres are not resolved very well, then certainly the more complex ones won't be, so I would first use instruments with less complex timbres.