I'm not a huge fan of the whole lo-fi recording/editing that's especially prevalent in early black and thrash metal genres so many of my favourite covers will be from more recent bands covering old classics. Many Bathory covers, especially, such as Emperor doing call from the grave, Cradle of filth covering bestial lust or the many a fine day to die covers (emperor, ereb altor, blackbraid, etc)

Also, Metallica has been covered a ton. And many of them are great. Not hard but yeah.. Revocation - dyer's eve or Vader - fight fire with fire

And ofc there's also the many cool pop song covers. Someone already mentioned CoB (rip Alexi) and there's Turisas - Rasputin or Avantasia - lay all your love on me or Nanowar's Norwegian reggaeton (which I think it's technically a parody rather than a cover, but amazing nonetheless) or Leprous - take on me or Elvenking - heaven is a place on earth