(Please see discussion before adding to this answer) Unfortunately I can't give you a real answer to this as it all comes down to taste unless you compare bands by record sales so instead, I suggest this should be a list of any (in your opinion) good death metal bands, including fusion and sub-genres such as melodeath, blackened death, and death/doom.
Death metal bands
Death
Melodic death metal bands
Dark Tranquility
At The Gates
Norther
Kalmah
In Flames (early stuff)
Amon Amarth (although it can be questioned whether this is melodeath or viking)
Ensiferum (same as for amon amarth)
Wintersun (ditto)
Skyfire
Mors Principium Est (weird fusion of melodeath with euro-industrial)
Children of Bodom (early stuff)
Technical death metal bands
Arsis
Meshuggah
Blackened death metal bands
Dissection
Behemoth
You
None other than Death itself.
helloween
Gothenburg's finest , In Flames. Likely In Flames. I would say the band Death itself. it is all just opinion anyway.
Children of Bodom isn't thrash metal. It's melodic death metal. They are influenced by thrash, though.
They are a heavy metal experimental rock band. Basically a band with a really heavy sound, but they mix various genres. From death metal to soul. But, alternative experimental metal suits them best.
This question can be a bit more ambiguous than you realise. Out of all the band which played Death Metal, Sepultura would be it. However, none of the albums they released during their Death Metal period would be particularly high grossing albums. Out of straightforward, true-to-form Death Metal bands, who haven't deviated away from it, Cannibal Corpse would be the band. As for the so-called Melodeath bands, they're irrelevant.
That depends on your own opinion, but the quality and skill also play a part in of what might be qualified as the "best" death metal bands. Here are some good death metal bands from different areas that I would advise.. That's just a small handful of some of the "best" bands.New York:ImmolationIncantationSuffocationMorpheus DescendsBaphometFlorida:Morbid AngelNocturnusMonstrosityMassacreObituaryCanada:GorgutsCryptopsyMartyrSweden:NihilistEntombed(early)DismemberHypocrisyTherionUnleashedSacramentum(early)GraveNorway:CadaverMolestedFinland:DemilichDemigodAmorphisSentencedAdramelechBelialNetherlands:PestilenceAsphyxGermany:Atrocity(early)BloodFleshcrawl
Korpiklaani is often thought to be the best folk metal band of all time.
Metallica, Slayer, Motorhead, Black Sabbath. Metallica and Megadeth.
In Flames, with reportedly 2.3 millions of records sold worldwide, as of May 2009.
"Iron Maiden are the best metal band and one of the best rock bands there has ever been" said by Lars Ulrich (Drummer of Metallica)