+2
Not much discussion here so I'll chip in. It's kinda hard to comment a list like this because 1) it's your personal opinion and 2) there's so many albums already. Anyways here's some of my opinions about your opinions.
I'm very happy about Bathory making your list even though my personal favorites are still missing at this point (them being Hammerheart and Blood Fire Death). Massively influential band (or person) and a proof that vision and desire are more important than technical skill.
I'm not a huge thrash metal fan. I have some favorites like Slayer, Metallica, Sabbat and Skyclad (if you can count them as thrash) but in general I mostly value it only as a transition from traditional metal to extreme metal. I've heard majority of the thrash albums on your list this far but in some cases I haven't listened to them in years.
I also don't think there's more clear "debut being the best" trend in thrash than in any other genre. Only clearly best debut that comes to mind is Annihilator's Alice in Hell; otherwise I actually prefer non-debuts from my favorites.
Helloween is influential for sure but I'm not really into German brand of power metal (their debut is their best album but it's more like speed metal). My taste in power metal is leaning more towards Southern Europe (Italy and Spain) and some bands that are little different from the norm like Sabaton or (old) Manowar.
About death metal. I used to listen quite a bit DM but as years go by more and more of it has started to sound dull. There are exceptions though and you've already mentioned one of them, Morbid Angel. Blessed Are the Sick is one of my absolute favorites and the album that converted me into harsh vocals. Altars of Madness isn't much worse (though it's kinda debatable whether it's their debut or not, Abominations of Desolation was recorded 3 years earlier but wasn't published until 1991 and it's also very good). I'd really need to give Death a proper chance though.
Albums on your list that could make my top-100: Holy Diver, The Eye, Altars of Madness, De Mysteriis dom Sathanas, Conspiracy, Heaven and Hell, Epicus Doomicus Metallicus, South of Heaven, Ride the Lightning.
And then short comments on few individual bands/albums.
Black Sabbath: You seem to love Tony Martin era. I think those albums are somewhat good but wouldn't count them among my favorites. They don't sound like Black Sabbath anymore to me.
Elvellon: Biggest surprise on the list to me. Never heard of them before and gave them a spin after their list entry. I'm sorry but to me they sound completely generic example of the genre and couple of weeks later I don't remember anything of that album.
King Diamond: We like the same albums. Of my three favorites by him only Abigail hasn't showed up. Those three are absolute killers.
Emperor: Their debut is good but I disagree about the later albums. All of them are good.
Virgin Steele: I've listened Invictus couple of times and while it's OK it's far too long. Also it's way too blatant Manowar ripoff to really impress me.