Call me Pat Boone, but I'm in a metal mood right now. My second metal thread in the past minute.
My problem with a lot of metal is the production. I find a lot of death metal ends up sounding like a bunch of mud thrown together. Or, as I stated in my "Does Black Metal suck" thread. Emperor's "Anthems to the Welkin At Dusk" has no presence. It's just a reverbed out mess.
Maybe it's the bassist in me talking, but I think THE most ignored factor in metal is the bass. It's often just mixed as low end with no bite (mids). Two examples that come instantly to mind that I own are Iced Earth's "Something Wicked This Way Comes" and Type O Negative's "World Coming Down." James MacDonough is almost all low, and is just this clunk that is on the track that totally clashes and doesn't mesh with the 45 guitar tracks Jon Schaffer lays down. Peter Steele (Though I wouldn't say it to his face

) has really great clean tone on the album, but the fuzz he has is more high end than anything. And he's fuzzed throughout most of the album. When I listen to it (Fantastic album BTW) I crank the bass boost and turn down the volume a bit and it sounds great.
But some of my favorite metal productions are...
Overkill - From the Underground and Below
Number one by far. And proof of my theory about a good bassy yet midrangey bass tone can make a metal mix sound awesome. D.D. Verni is kick ass to begin with, so that helps. But I consider this the greatest metal bass tone ever. And everything else sounds perfect. Great drum sound. The perfect pure metal album. Not death metal, not black metal, just kick you in your face/drink beer/kill babies.... well maybe not kill babies... metal. I'm not too fond of the 80s stuff though, production or music, too cheese.
Cannibal Corpse - Bloodthirsty
Great metal sound, it does the death metal with only enough mud to make them sound like C.C.
Cryptopsy - Whisper Surpremacy
The best sounding over the top death metal album I've ever heard. Everything is nice and crunchy, sometimes the guitar sounds a bit weird, but, it all blends really well.
System of a Down - Toxicity
Not my favorite album, but I do respect Rick Ruben on this one. It's not as heavy sounding as the first one, but it's got it's own quirky metal style. And everything breathes very well, there are overdubbs but not a trace of mud, it's all very very crisp.
Sepultura - Roots
The production makes the album sound heavier that it should somehow. Which is very cool.
Metallica - The Black album
Production wise, Metallica's best.
Prong - Rude Awakening
This is when Prong fully embraced their Industrial sound, while still maintaining that ultra heavy guitar sound. Bass and Drums and Fake Drums create this awesome heavy groove machine.
Pantera - Vulgar Display of Power
Awesome.