The original mix of Nevermore's Enemies Of Reality before Andy remixed it. Its sole redeeming virtue (besides their playing, of course!) was that it sounded to me like a pretty decent representation of what a Mesa Rectifier actually sounds like in a room. Points for that. But the distorting vocals and buried, lo-fi drums were reprehensible.
I remember Nile's In the Catacombs Of Nephren-Ka being really, really unbalanced. I haven't listened to it in a long time, so I might be off here, but I remember really thin sounding guitars that were seriously overpowered by really bad sounding drums.
Morbid Angel's Heretic is pretty unpleasant. All V-Amp and JCM 900 DIed, IIRC, with thin, wonky sounding drums. Sounds like a demo; just flat, no depth or body whatsoever.
Honorable mention to Pain's Rebirth for having one of the harshest, ear-fatiguing guitar tones I've ever heard. Peter Tägtgren's vocals sound really off-key to me, too.
IMO, St. Anger and a lot of black metal should be excluded from the discussion. Those are
intentionally bad mixes as an artistic choice.
I don't think Death Magnetic is that bad at all, really; the mastering is pants- it's
really fatiguing- but the mix isn't terrible. My biggest complaint there is that the kick gets buried. Lars' snare has been mixed way loud since Justice (which I don't mind the mix of either- it's not "good" in the traditional sense, but suits the mood of the music), and sounds fine IMO. Well, when it's not clipping.