Top Ten Melodic Death Metal Subgenres/Styles

This is a rare style of MDM combining euphonious melodies with mad and impressive technicity, like you know from Necrophagist maybe. In Death Metal the phrase "Technical" and "Melodic" seem to exclude eachother, so there are not many bands who really pull this off very well to merge these styles.
But the result is very overwhelming MDM, with furious guitar play throughout while not sounding too progressive.

This includes:
Quo Vadis
Arsis
Inferi
Allegaeon

I find myself liking technical versions of all metal subgenres, not only death metal. I like a lot 2 bands mentioned by you - Necrophagist and Quo Vadis. It doesn't mean I don't like bands from the other melodeath styles though. Amon Amarth got me into death metal and I still like them, and so on.

This Old School Style of MDM is that one, which first emerged from Death Metal with not much other influences. Its that classic Gothenburg sound with just e-guitars, bass, drums and harsh vocals, which make heavy but very harmonical swedish melodies with occasional acoustic parts and very few clean vocal parts (if not no cleans at all) that In Flames and Dark Tranquillity pioneered.
At The Gates and Carcass created at the same time a similar, but a bit different style of MDM, you'll find under "Thrashy Melodic Death".

This includes:
Amon Amarth
Hypocrisy
In Flames (1994-2000)
Dark Tranquillity (1993-1997)
Soilwork (1998-2000)

Progressive Melodic Death Metal is a very interesting genre, which is sometimes a little experimental and features a more complex and "pseudo-classical" composition. Complex rythms, unsusual time signatures and atypical song structures are very common, while not becoming too experimental/avant-garde like creating weird noises and unusual sounds.

This includes:
In Mourning
Persefone
Edge Of Sanity
Opeth
Dan Swanö's Solo Project

This is a style of Melodic Death Metal focusing on riffs and soloing in the vein of Thrash Metal. It has this typical pounding and sometimes groovy guitar work, but in a very melodic matter. Guitar solos are very present, some bands even play guitar duo battles. Usually it doesn't feature any other instruments besides guitars, bass and drums.

This includes:
Arch Enemy
Arghoslent
Carcass (1993 - 2013)
Exmortus
At The Gates (1994-2014)

It's the style of death metal which make a great (sometimes doomy) atmosphere. Its mostly very beautiful in sound, while it doesn't necessarily have to be slow all the time. Bands make prudent/sparsely use of Keys and Symphonical elements, which help building up a certain atmosphere, but don't get in the foreground.

This includes:
Insomnium
Omnium Gatherum
Be'Lakor
Swallow The Sun
Garden Of Shadows
Amorphis (1994)

This is a very epic and beautiful style of MDM, lead by many symphonic elements, orchestra sounds and classical instruments, who are in the foreground. The symphonic elements work often together with the guitars and are in great complementary with the guitars. Mostly it has slower and also faster parts, which can be melancholic at times. Neo-classical elements also can be found.

This includes:
Eternal Tears Of Sorrow
Septicflesh
Imperial Circus Dead Decadence
Dark Lunacy

This is a very beautiful and epic style of MDM, lead by many symphonic elements, orchestra sounds and classical instruments, who are in the foreground. The symphonic elements work often together with the guitars and are in great complenetary with the guitars. Mostly it has slower and also faster parts, which can be melancholic at times. Neo-classical elements also can be found.

This includes:
Eternal Tears Of Sorrow
Septicflesh
Imperial Circus Dead Decadence
Dark Lunacy
Serpent [JPN]
Die Apokalyptischen Reiter (2000)

As you can guess, this is Power Metal based MDM, but doesn't necessarily need clean vocals. The feisty guitar riffing and the use of of Keyboards are the decisive factor. It can be similar to Symphonic MDM sometimes, but lacks more of the symphonic elements and has often a somehow "cheerful" atmosphere. The Keyboard-Leads are in complenetary with the guitars, often in solos as well!

This includes:
Children Of Bodom
Kalmah
Norther
Skyfire
Wintersun

This sounds paradox, but actually exists. It is a very rare style, which is actually more pure Death Metal, but has some melodies in it. The gutturals are very low, and the riffs are typically way heavier than usual MDM.

This includes:
Intestine Baalism
Acephalus

This is a very catchy, but still very heavy, MDM style. It's a style which adds to the original Melodic Death elements even more melody and variety. Clean vocals are very present (most times cleans and gutturals live in symbiosis and complement eachother). This style is very open for different musical elements and often features classical instruments (like pianos) and also electronic influences (like synths).

This includes:
Scar Symmetry
Amorphis (1996-2015)
Soilwork (2001-2015)
Dark Tranquillity (1999-2013)
In Flames (2002, 2006)

I'd also put Mors Principium Est in here, the description "modern style" suits their overall sound better than the other in my opinion.

This is mostly a fusion of Melodic Death Metal and Metalcore. Bands, who play this style, sparsely/rarely use breakdowns (or if not no breakdowns at all). It's more like Melodic Death Metal with a different vocal style, featuring typical metalcore/deathcore-similar gutturals and often also clean vocals.

This includes:
Sylosis
The Agonist
Heaven Shall Burn

I would also put The Black Dahlia Murder in here. While their instrumental work is undeniably Melodic Death Metal in the heavier style, their unique vocal style often get them confused with Deathcore.