Seriously - it depends who you ask and what they like. A rap fan may never have seen Nirvana or U2 or Pink Floyd covers - a rock fan may be oblivious to Tupac or Eminem sleeves - it's subjective.
not a big fan of their music but the cover of Regeneration by the Divine Comedy is superb. also the new Sigur Ros album 'Takk' comes with a lovely CD cover.