Someone can explain to me who would consider yet another "lead artist/interpreter possibly with his instrument in a mundane situation" a great cover? Anyway, something more objective, interesting and rare is when the cover really matches the content (sonically, not lyrically, that's too easy). Some examples:
Enslaved - (1994) Frost
Indeed their most icy and minimalist album.
Atrocity - (1990) Hallucinations
Dark and psychedelic. One has to listen to it a few times to get how good the cover is.
Crowbar - (1991) Obedience thru Suffering
As crushing as the rough painting and what it depicts imply.
Well, it's a bit easy for extreme metal, since the style is so visceral by nature.