What makes Wunderlich's a supreme voice for me is the combination of thrilling masculine tone and strength, with an underlying pathos, an earnest quality. This was, no doubt, partly due to his youth - he died at only 35 years old (due to an accident - falling down stairs), which is barely reaching maturity for a male singer.
Technically, Wunderlich was noted for his breath control, giving him the ability to sing long phrases and (this is much more difficult than might be imagined) to increase or decrease volume while holding a high note - without either going-off that note (losing intonation) or breaking the continuity of vibrato.
This is shown to great effect in the notoriously tricky Il Mio Tesoro from Mozart's Don Giovanni, sung live. Listen for the long, single breath passages with rapid runs up and down the scale.
This aria demonstrates the 'heroic' quality of Wunderlich's voice - which is unusual in this type of lyric tenor.
Being a live performance; Wunderlich snatches very quick breath in the middle of the very longest passage, which enables him to slow down and expand the last part of it.
Below is have a studio recording of the same aria, sung in German translation, in which he sings the phrase right through with no trouble at all - but this is usually not possible in live performance where the singer is often tired by the stage movements and acting.
If you are impatient to hear it, jump straight to 1:45. It makes me feel a bit faint just listening...