So, a note. There's two kinds of covers. Covers where the band tried to do exactly what the band they're covering did. And covers where bands made it their own.

If you want a whole album of some excellent examples of the former, look no further than BTBAM's "The Anatomy Of". Whole album is covers.

If you want the latter... What comes to mind is Children of Bodom. They did like 30+ covers that metal fans love. Off the top of my head:

Children of Bodom - Hellion (W.A.S.P)

Children of Bodom - Oops I did it again. (Brittney Spears)

Children Of Bodom - Somebody Put Something In My Drink (Ramones)

Then we got

In flames - Everything Counts (Depeche Mode)

In Flames - Hurt (Trent Reznor)

I'll keep updating this. More are coming to mind.

System of a down - Snowblind (Black Sabbath)

Shadows Fall - Bark at the moon (Black Sabbath)

Dark Tranquillity - Acacia Avenue (Iron Maiden)

Soilwork - Burn (Deep Purple)

Ooh, just remembered:

Avenged Sevenfold - Retrovertigo (Mr. Bungle)

If dwarf metal counts then it's got to be the Wind Rose cover of Diggy Diggy Hole
Original

Every time we touch by Electric Callboy

Napalm Death - Nazi Punks Fuck Off

Anthrax - got the time

Original is a Joe Jackson bop

I very much enjoy Otep's cover of Royals by Lorde. The angst of the lyrics match up really well with the human embodiment of angst known as Otep.

Some current favorites: Judas - Rage of Light (Lady Gaga cover) https://www.youtube.com/watch?v=Z7cvquM3zNc&t=287

The entire album "Epica vs Attack on Titan Songs" by Epica

Powerwolf's multiple bonus albums where they have other bands cover them or collab. Interludium, Communio Lupatem. This gave us the awesome Battle Beast cover of Resurrection by Erection, where you can tell they were having a blast.

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

Allegaeon covering Roundabout is amazing. Allegaeon in general is an amazing underrated band with some phenomenal covers too

Equilibrium - Himmelsrand (skyrim theme song)

I think In This Moment did a fantastic cover of Call Me by Blondie

https://song.link/t/55210608

Also, they're kind of toward the metal end of hard rock, but Halestorm did a great version of Lady Gaga's Bad Romance

https://song.link/t/5935702

I really like Ensiferum - Battery (Metallica)

Caleb Canatheviphth's ‘Hammer Smashed Face’

Eddy Jaber's ‘True black metal Bach choral : O haupt voll blut und wunden (BWV244)’

Zachary J.'s ‘Night on Bald Mountain’ (not AI, despite the image)

Therion did an entire album and an EP of covers of 60s' French pop songs: e.g. ‘Nous ne sommes pas des anges’ and ‘L'amour est bleu (mellow version)’ or the heavy version. They also have gorgeous cover of ‘Summer Night City’.

Le Scrawl's ‘Ring, Ring, Ring’

Sasha Garcia's ‘The Cereal Mouse’, if this counts; and also his ‘Mutilate the Stillborn’

Cover: Mercenary - Music Non Stop

Original: Kraftwerk - Music Non Stop

Leo Morachioli's cover of toss a coin to you witcher

And his final countdown small drums solo is pure joy. Watch the video for effect think his YouTube channel is called frog leap

One of my all-time-favorite metal covers is of The Postal Service - Such Great Heights, which was covered by Confide. (Note: The Metal starts around 40 seconds in.) It's blend of Metalcore and Screamo made this cover of this song made it stand out once again for me.

Andre Antunes’ metal renditions of:

Sanson Ki Mala Pe by Nusrat Fateh Ali Khan

Bulgarian Folk Song Ergen Deda

Blümchen - Herz an Herz & Roxette - The Looks (Lord of the Lost)

Tragedy, in general. Death to false disco metal is a great album.

A lot and i can't think of them all atm. But here's one. I'm at work and brain hurts

Revolting Cocks - Do Ya Think Im Sexy.

God Rest ye Merry Gentlemen - Dio & Iommi

I know it's a Christmas song, but they really do make it epic!

I have one that's a metal -> metal cover that I love: Ember Falls - Cloud Connected a cover of In Flames

I keep coming back to Sienna Skies' cover of Fireflies (originally by Owl City).