DUDE! JOSH!! HOW HAVE I NOT EVER SEEN THIS POST?!?!
BTBAM is my ALL TIME FAVORITE BAND EVER!!
I've seen them twice (once on the progressive nation tour with 3, Opeth, and Dream Theater, and the second with The Faceless (FREAKING AWESOME LIVE), 3 Inches of Blood, and In Flames (also very good)). They're amazing. Have you heard the Great Misdirect yet? It's awesome.
I really like Incubus too, they're very unique sounding.
Scar Symmetry is good too. Yay Swedish metal! There isn't much out there that has the same mix of sounds as they do. However, their singer left, so their new album isn't quite the same
Opeth (yay Sweden, again) are also an amazing band. They were my favorite before I found BTBAM.
Katatonia is consistently good. I've heard positive things about their new album, but haven't actually heard it yet.
Textures is an under-rated band. They're really really good. From the Netherlands, of all places lol.
(and I'm copy/pasting something as well for my contribution lol)
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I think the only brutal death metal I really like is a Belgian band called Aborted. Their last two albums, Slaughter & Apparatus: A Methodical Overture, and Strychnine.213 are really quite good. Focused more on the complex instrumental work than the vocals, while still being brutally heavy... Also the vocals are pretty original. They range from traditional death metal growls to higher pitched screaming... I actually really like them.
Since you're well aware of In Flames and Opeth, and that genre/region of music, I'm sure you know about Arch Enemy. Melodic Death Metal, they're also very good. HOT girl singer, Angela Gossow [Sweden ftw lol]. I'm a big fan of Michael Ammott. Before he started Arch Enemy, he was in a band called Carcass. Carcass's album Heartwork came out in 1994, and truly redefined melodic death metal. It's a classic, a very very good album. I highly recommend it.
Arsis is very technical death metal, very good. It's all pretty much one guy writing everything. I think... haha. A diamond for disease, and We are the Nightmare are both really good albums. I've also heard A celebration of guilt is good too...
Daylight Dies is an obscure band from like Virginia or something. Not many people know about them, but given your opinions of Katatonia and Opeth, I would definitely recommend them. Their best album is Dismantling Devotion, but you can't really go wrong with anything else. They are slow, melodic, depressing, with death metal growls. They're categorized as "melancholy melodic death metal" which is pretty accurate. You always get the feeling that they have so much power, that they're capable of something so much heavier than what they play. It adds a unique tension to the music.
Despised Icon is only slightly less technical than Arsis, but still good, and sounds almost similar, with the exception of the vocals. They've kind of found a unique niche in that they have two dedicated vocalists, one for high screams, and one for low growls. I have only heard The Healing Process, so I don't know what their newer albums sound like, but it's most likely quite good.
The Faceless, opening act for BTBAM when I saw them in october, are really really heavy, pretty technical, and really good. They've only been around for two albums, but both are decent. If you get an opportunity to see them live, I would. They play very tight and really well.
I've just recently started listening to Gojira, and I like what I've heard. I don't know enough about their library to point you in the best direction, but I've liked everything I've heard so far. Randy Blythe from Lamb of God did guest vocals on "Adoration for None" off of The Way of All Flesh, which is pretty interesting. (Lamb of God is my number three favorite band, after BTBAM and Opeth. Kind of a different style, but I really like it)
The alphabetical order of my library (if you haven't noticed yet haha) leads me next to Hacride. I have never met another person who has heard of them. They're an obscure French metal band, but they are very very good. The album Lazarus is an epic masterpiece. The first song is over 15 minutes long, but hardly feels like it. It's sort of a cross between Opeth and something else... not really sure. They take their time getting places kind of like Daylight Dies, but at the same time, they aren't afraid to just blast your face off. The music video for Perturbed (first song off of Amoeba) is one of my favorite music videos.
The Human Abstract is kind of interesting. You have to listen to Midheaven at least once, just to understand. Every song is practically a different genre, often switching mid-song. It's really interesting. It's all good, it's just... interesting how they put it all together.
Lamb of God is growing more and more mainstream without changing their sound too much, and it's easy to see why. They're really easy to get into, and there's not much else that sounds quite like them. Not sure why, but... yeah.
Obscura is another fairly new technical death metal band. Their debut (and only album so far), Cosmogenesis, is very well written, well executed, and very very good. The bass is mixed quite prominently so it's cool to hear it pop out of the mix whenever it plays a high note. It adds a unique dimension to their sound.