@Bold my favorite!

Can listen to every one of their songs and still enjoy it even after 10+ years of no new content.

Hmm, I guess you could check out the recommended albums list, but you may already know most of those albums.

Agreed in some respects. Reckoning Night is simply a masterpiece, one of my favorite all time albums. Just has the perfect balance of speed metal, classic power metal, and more melodic stuff with a hint of progressive stuff which suits the band best I think. After that they leaned too heavily on the progressive stuff and tried to experient with some weird styles/instruments that was hit and miss with me. 

I do have a soft spot for Unia as this was roughly when I got into the band, so there's a certian sentimentality with it, plus I do like some of the more progressive tracks on that album. For some odd reason much of Stones Grow her Name resonates with me too despite most fans seemingly hating it - I think it's just because of the weird, off-the-wall tracks and diversity in the style of music on that album, as well as some songs with catchy hooks. The only albums I don't really care for as a whole are Days of Grays (a lot of whiny songs and ballads) and their latest one Ninth Hour (few catchy tracks, and most songs just seem to blend together on account of the constantly shifting melodies and structures). Pariah's Child is pretty lukewarm for me to, though it does have a few tracks I enjoy. 

But yeah as a whole, early Sonata > current/post-Unia Sonata no question.

Metal is actually an insanely complex and in depth style of music in terms of structure/melody/rhythm, etc hence why so many sub genres have branched off. There's no written rulebook carved in stone as to what defines a genre and which band falls under which one (a lot of times metalheads debate which genre a band fall under), though usually a band trends towards at least one or two that most people agree on. And there are usually basic "guidelines" of characteristics and noticable differences between a lot of sub genres. Part of it is metalheads are typically pretty anal and detail-oriented so we thus like to compartmentalize and categorize bands.

A lot of times the difference is pretty apparent even to the casual listener - power metal is usually high-pitched vocals, dungeons & dragons lyrics, and keyboards, folk metal will have traditional folk instruments for instance, doom metal is very slow moving and low-pitched, goth or symphonic will often have female vocals and a more operatic/classical music sound..

But a lot of times the differences are more subtle. The more extreme style for example - death, black, and thrash, usually abandon traditional song structures and catchy "hooks", and thus similarly sound like "noise" to people - though they are actually more complex in some ways and do have differences that most metalheads will pick up on. Death metal for instance contains blastbeats, growling, and gore-related lyrics, black metal being more theatrical elements, shreiking vocals, crappier production, and satanic lyrics, while thrash focuses a lot on the strength of the riffing, speed, double bass drums, and lyrics dealing with death, politics or anarchy.  

Nu-metal? That's debated by the "diehards" whether that even belongs in the umbrella genre of metal, as it is very mainstream and pop-esque in nature, and incorporates elements of rap and/or techno. To me it has its place in some ways (hell numetal was the gateway genre for me and many others to tumble down the rabbit hole of more complex and heavy genres), but it still kind of has one foot in the pop/rock arena..

Metal is pretty much my favourite music genre, been listening to it for almost 20 years now. Generally I'd say that I listen to almost all kinds of metal music, up to a point. I even wrote my bachelor's thesis of history on how British and German metal bands in the 80's discussed and treated the cold war in their lyrics.

Nu Metal is like Djent. It's no real genre but a label. If you listen to bands that are put into those "genres", they are all pretty different which normally would mean another genre in metal. Singing style, guitar tuning, complexity and mood mostly differs but media will still call it Nu Metal/Djent.

I'm into bands like Sybreed, The Interbeing, DVSR. And I came all the way from Black Sabbath, Rainbow, etc. thanks to my dad.

I didn't include nu metal, as many people don't consider it to be metal, so I just avoided that can of worms altogether.

Just gonna list my favorite bands and what I think are their masterpiece albums, which everybody should listen to.

Metallica (Masterpiece albums: Ride the Lightning, Master of Puppets, ...And Justice for All) Except for St. Anger and Lulu, I like almost everything they have done. Their 80's output is sick though.

Judas Priest (Masterpiece album: Painkiller) Might be my favorite band, they have made an abundance of great albums, probably more than 10, but Painkiller clearly stands as their magnum opus.

Iron Maiden: I'm not really sure if they have made a true masterpiece, but they have come close multiple times throughout their carreer. With few exceptions (No Prayer for the Dying, Fear of the Dark, Dance of the Death) they have made excellent albums consistently.

Queensrÿche (Masterpiece album: Operation: Mindcrime) They made a string of excellent albums throughout the 80s up until and including 1994's Promised Land. Then they almost became irrelevant for 20 years until a new singer started a second golden age for them.

Black Sabbath: (Masterpiece album: Paranoid) They made so many great albums. The first 6 with Ozzy and the 3 with Dio really stand out. My favorite is still Paranoid though.

Megadeth (Masterpiece album: Peace Sells, but who's Buying) I don't like their recent stuff all that much, but they were definitely big in the 80's and early 90's. Most people will mention Rust in Peace as their most important album, and while it contains arguably their 3 greatest songs, it is a very inconsistent (although still great) album in my eyes. I much prefer the more raw Peace Sells, but who's Buying album.

Savatage: Just a great band with arguably the greatest metal singer ever (at least tied with Halford). They might not have a masterpiece album, but at least for terrific ones: Hall of the Mountain King, Gutter Ballet, Street: A Rock Opera and Edge of Thorns

Blaze Bayley (Masterpiece album: Silicon Messiah) I highly prefer his solo output to that of the more popular Iron Maiden vocalist (Bruce Dickinson), and dare I say, I prefer his voice as well. Silicon Messiah is his masterpiece, but any of his first 5 albums are amazing.

Dream Theater (Masterpiece albums: Images and Words, Metropolis pt II: Scenes from a Memory) They were something truly special in the 90's. They are still good, but they tend to focus too much on technique.

Pantera: Really a raw and unique band, I think Vulgar Display of Power and Far Beyond Driven are terrific albums, though not quite masterpieces.

Deep Purple: Not always considered metal, but I think they should. In Rock and Machine Head are terrific albums.

Other metal bands I love, though to a lesser extent:
Helloween
Motörhead
Dio
Rainbow
Testament
Children of Bodom

And then artists that occationally have made something I care about or bands I can stomach in smaller doses, though I don't consider myself a fan:
Slayer
Ozzy Osbourne
Tool
System of a Down
Diamond Head
Iced Earth
Bruce Dickinson
Rob Halford
Opeth

I don't consider most of it to be metal.  It had chunky guitars (sometimes) but took too much influence from pop and hip hop to really be accepted by the community.  And I don't mean to say that any bands with rapping can't be metal. Stuck Mojo and Body Count kind of prove that.

I've been listening to a shit ton of death metal and black metal recently, thanks to Spotify I've downloaded a ton of albums, still need to listen to a lot of it!