Something with as big a dynamic range as possible.
When I was a test engineer for Arcam ( a good few years ago now) we used a Philips test disc that had all sorts on it, ranging from very boomy spoken word to incrediably dynamic and detailed classical recordings.
Also, bear in mind (as some have said) that hi-fi is very subjective and unlike the world of studio equipement for example, where linearity of response is king.
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My dissertation was very nearly about the ideal linear response of spherical speaker inclosures.. they would have sounded nice in theory but making a rigid sphere of sufficient density isn’t easy… why the flip did I go into computing and not take up that research degree at York, mutter, grumble, moan
#end rant