In no particular order, aside from number one, here's my stab at a top 20 "all time favorites" list:
Opeth - the only place for this band is at the top of my list. I was lucky enough to catch them kind of early on, at a formative time for my music taste and songwriting ideas. Tranquil melancholy, intuitive melodies, and dark aggression were held together by abstract songwriting that always had a sense of grace and clarity. I don't care for their last few recordings, but their first eight albums are all gold in my book. The first one I heard, and the one that had the biggest impact on me, was Morningrise.
Katatonia - although they've covered a good amount of stylistic ground in their 20+ years of development, from the raw, blackened melodic doom (dark metal, you say?) of their early albums to the polished modern prog-gloom sad rock of their latest, their heartfelt sense of melody remains a constant. Hard to pick one album out of a pretty big and somewhat varied discography; if forced to do so, I'd pick the one that won me over: Tonight's Decision.
Type O Negative - Gothic rock, punk aggression, 60s psychedelia, sprawling song structures, sarcastic humor, sincere explorations of shame and abandonment issues. Nobody else had Pete's voice; nobody could write songs like Pete. RIP Pete. Desert island album: World Coming Down.
My Dying Bride - another unique combination of aggression and melancholy melodies. I'm sensing a theme here. Doom-death with a growing gothic tinge, evocative guitar lines, long songs full of weird and often awkward transitions. I don't love every single album, but they're still doing good work. The definitive classic: Turn Loose The Swans.
Rush - my first "favorite" band as a kid, and one I've grown to appreciate even more as I get older. I've consistently enjoyed these guys since I first heard them almost thirty years ago. Their musicianship gives us all something to aspire to; Geddy Lee's polarizing vocals stand guard at the gate, keeping out all but the true seekers. Or something. The archetypal ur-text of prog rock in my internal mental library: Moving Pictures.
Soundgarden - ah, Chris Cornell. This guy could put anything from a delicate, intricate melody to a furious howl over pretty much any clunky odd-meter riff or weird chord change, and make it sound natural. Matt Cameron could make it groove. Kim Thayil could write it to begin with. And Ben Shepherd could... well, he played the bass. (Ok, sorry, he wrote stuff too, and also sang backup, and really helped expand their sense of what was possible in songwriting when he became Hiro Yamamoto's permanent replacement in 1990-91.) These guys could - and did - make memorable themes out of almost anything. One of the big superstars of "grunge", with a wide-ranging willingness to explore. Even the dad rock of their comeback album had a lot of great moments of genuine inspiration. Cornell's suicide hit me like a ton of bricks. Most unique statement: Superunknown.
Alice In Chains - while we're on "grunge", the story of my life wouldn't be complete without the nihilistic, drug-laden emotional rollercoaster of AIC's early albums. Honest riffing, clear and deceptively simple songwriting, and a raw, fiery edge to their trademark dual vocal attack, opening a door on the inner conflict hinted at by their cryptic lyrics. Anger, sadness, boredom, and the occasional epiphany, always with the sense of something horrible lurking around the corner. Hardest gut punch: Dirt.
Swans - primitive, abrasive fury? Proto-industrial bludgeoning? Spiteful declamations? Lush, reverb-drenched atmospheres? Gothic anthems of the bombastic or meditative natures? Unnerving soundscapes? Pains and pleasures to frighten the uninitiated, magic carpets to transport the half-awake, blankets to smother the enlightened? Anyway I like this band a lot. They've been all over the place since their inception in the early 80s, and one can't encapsulate that kind of career in a single album; but if one wants to try, one might listen to Children Of God. (If one wants to forgo the theatrics and just hit things for fun, one would be advised to listen to Filth instead.)
Enslaved - A couple teenagers from Norway in the early 1990s like the sound of black metal, but would rather write about Viking stuff than Satan. One of them turns out to be a prolific songwriting genius who develops a unique fusion of black and folk metal with psych rock and 70s prog, over the course of fourteen albums and counting. There's a mysticism in this music, the sense that you're going on a journey, or that the world is going on a journey around you; and energy is always present, even if it bubbles under the surface. Favorite mystical journey: Below The Lights.
Death - one of death metal's pioneers since their demo recordings (as Mantas) in the early 80s. Various lineups (including some serious A-list talent) under the direction of guitarist/songwriter/frontman Chuck Schuldiner put out seven influential and increasingly eclectic albums in an eleven-year period, before "disbanding" to focus on new music with a different vocal style. Chuck's distinctive riffing and soaring melodies helped define prog death for me, and probably a generation of other metalheads. A difficult choice between any of the last four albums for me, but the one that first opened my eyes was 1993's Individual Thought Patterns.
Isis - another band that opened me up to a new way of thinking about songwriting. Big, open structures, layers of different textures, lush atmosphere, and carefully considered dynamic shifts across a spectrum from sparse post-rock to harsh, abrasive sludge. When you don't know what to call it, call it "post-metal"... I was a fan of the early, noisy stuff, but they really hit me with 2002's Oceanic.
Akercocke - blackened progressive death metal with tons of great melodies, varied and passionate vocals, and tasteful synth/electronic elements, at the service of enjoyable, memorable, cohesive songs that often pull together my favorite parts of disparate influences as if by magic; what's not to love? Their early stuff is vicious, and their comeback album from a couple of years ago was stellar, but the one that cemented them in my head as a favorite was 2005's Words That Go Unspoken, Deeds That Go Undone.
Gorguts - a dark, uncompromising death metal powerhouse, from their enjoyable early albums through their idiosyncratic, twisted late 90s/early 2000s material to the gorgeous intricacies of their newest recordings. This band tried things few others had even thought of, and managed to succeed. I absolutely love what they've done since their new lineup formed, and in a vacuum I might prefer to listen to it, but in the context of an "all time favorites" list, my pick has to be 1998's seminal Obscura.
Amorphis - a cold wind blew down from Finland when I was fourteen, carrying with it a sense of other possibilities in death metal aside from the meat-and-potatoes pummeling I'd gotten into - stark, angular melodies, slower rhythms, brooding gloom, cultural context. They'd shift gears pretty quickly, leaving death metal - and me - behind, but (for my taste) their debut is one of DM's pillars, wrapping most of what I love about the Finnish scene into a simmering yet catchy package: The Karelian Isthmus.
Queensryche - those vocal melodies! Those memorable riffs. That tasteful lead guitar work and skillful songwriting. I've loved their music since I was ten or eleven, I like their first three albums as much as anything above, and I used to jam the shit out of even the two after that. Hugely important to my musical development. And yet I don't care much for the vast majority of what they've done, including nearly everything from the past thirty years, and a full two-thirds of their output is steaming turds. Ah well. Forget that they didn't break up in 1990, and focus on their cinematic narrative concept prog-metal masterpiece, Operation: Mindcrime.
Cradle Of Filth - years of actual filth can't obscure the ambition and brilliance of their early stuff, particularly the second, third, and fourth albums. Interesting melodies, intense drum work, detailed and wide-ranging songs that turn on a dime, topped off with Dani's unique shrieks and dense lyrics. Almost a toss-up here, but I have to go with the grandiose, impressively well-realized black/death/gothic/symphonic synthesis of Cruelty And The Beast.
Nevermore - this band had great technical chops and real chemistry, which tends to get a little lost in the modern polish of their recordings. Jeff Loomis could (can?) write a mean riff, and he's one of the few flashy soloists whose playing I not only admire, but actually enjoy. Warrel Dane's haunting vocal tone, forceful delivery, thoughtful lyrics, and highly personalized melody and harmony ideas were the centerpiece of their songwriting. Brilliant inflection points were scattered throughout. Even their weaker music was accomplished and engaging; at their best, they were transcendent. Most heartfelt and cohesive album: Dreaming Neon Black.
Meshuggah - their influence on modern metal, and on me, can't be denied. It's easy to think these guys had that one idea, that one time, and figured out how to make a career out of it. Their big chunky polyrhythms and half-tone/whole-tone lead lines gave rise to a lot of tiresome ripoffs; I have to admit that I hate most of the bands they directly inspired. I don't play their stuff much these days. But when they first hit their stride, they sounded like nothing else, pulling together rhythms and textures with a sense of purpose that blew teenage Alabaster out of the water. Often imitated, never duplicated. For clarity, textural variety, and focus in songwriting, the untouchable gem in my book is their second album, Destroy Erase Improve.
Dark Tranquillity - for a little while, DT was my favorite melodic metal band in the world. Well-differentiated songs full of awesome guitar harmonies, and bold twists and turns that may as well have leapt from the head of Zeus. It couldn't last; they "streamlined" their third album into a passable answer to contemporary At The Gates before making a career out of music that I can't stand. They almost shouldn't be here. But their second album is my favorite thing to come out of the Gothenburg scene, and it's still relevant to me today: 1995's majestic The Gallery.
Metallica - ugh, I really have to put Metallica on this list. I don't want to; like a few of the other bands I listed, and to a greater degree than most, they went tits up, jumped the shark, shat the fucking bed. Not only that, but I hardly ever listen to them anymore, maybe once every year or two. And yet. And yet. Wow, these guys were a bolt of lightning. They put out the first metal album I owned, and they were the first "heavy" band I liked. They wrote the first guitar part I figured out by ear. They played the first concert I went to. I saved up money working all summer to buy legit tapes of their albums and a stereo to play them on. The riffs, the vocals, the songs... it's all there at the foundation of my musical journey. Maybe the most important band in my library. That's probably true for countless other people as well. Out of the albums I still enjoy, I'd have to choose 1988's bitter, intense, sometimes unexpectedly personal ...And Justice For All.
In a different year, under different lighting, any of the following bands might have made the list:
Faith No More
At The Gates
Sisters Of Mercy