I second that motion. Absolutely no fucking haircut bands!
Rather than bothering with Deathcore, why not return to what spawned it? When I got into death metal originally it was through MDM (Melodic Death Metal). Some of what people have already been suggesting in this thread is rather close to that (Opeth particularly). Some other ones (older and somewhat less poppiers stuff) that I used to dig back in the day that you might be interested in:
Arch Enemy - Angelclaw: Pre Angela Gossow Arch-Enemy is fantastic for reasons that have nothing to do with the singer. This song is pure guitar heroics, without ever tipping over the edge into malmsteenesque wankery.
At The Gates - Slaughter of the Soul: The memory of this band becomes more and more tainted as more and more metalcore and deathcore bands produce third-hand rip-offs of their sound, but this album is stll classic and I'll REFUSE HUGS to anyone who says otherwise
Carcass - Heartwork: I am actually pretty shocked that this is the first time they have come up in this thread. Carcass are
the fucking business. If you like this track you should also try some of their earlier (superior) stuff. Particularly indispensible classics include
Corporal Jigsore Quandry,
Incarnated Solvent Abuse and every single track on Reek of Putrefaction, an album that has inspired probably more blatant clones than just about anything else in the history of metal. Don't bother with the album Swansong; even the band practically pretends it doesn't exist these days.
Edge of Sanity - Twilight: EoS are a pretty key band. They're one of the key pioneers in the early mixing of death metal with elements of other musical genres, they're one of the first death metal bands to write songs about relationships that didn't involve someone getting stabbed in the genitals, and they had the benefit of at least two genius level songwriters, including the irrepressibly brilliant Dan Swano. Plus they are just good, which is actually what matters in the end.
In Flames - Moonshield: This is from the second-to-last In Flames album that is really worth buying. After this and the follow-up Whoracle the band completed their transformation into an absolute, painfully embarrassing joke who I deeply wish would just vanish from existence. Old stuff is alright though; early-mid 90's In Flames is probably the 'perfect' MDM. But seriously, even Whoracle is getting dodgy, and anything after that is pure dogshit*.
*I originally wrote 'pure goatvomt', but then I remembered that
goatvomit is awesome.