I really like the sounds Peter Tagtgren (Hypocrisy, Pain) gets both as a recording engineer and mixing. He shows up so much on late 90's swedish extreme metal and the sound is super recognizable. Even his more recent albums retain a more old-school sound on guitars, which I really appreciate given that I dislike the "modern metal" guitar sound. Also, the bass guitars on several of the albums he's produced is crazy. Plus, I really like the drum sounds he's gotten over the years, particularly the kick drum sound that shows up on most of the Amon Amarth albums he's recorded and mixed.
I guess Jens Bogren also should get a mention, as he's been on almost every big metal album for the past decade or two, including Opeth and Be'lakor, which are my 2 favorite bands right now