Ecthelion

41. Mercyful Fate - Don't Break the Oath
One has to wonder what proper metal compendium would lack this album and this band. Hailing from Denmark, this band's brand of heavy metal not only had an atmosphere rooted in the occult but had the musical substance to back it up. Whether it be the infectious rhythms from bassist Timi Hansen and drummer Kim Ruzz, the dramatically sinister melodies and epic solos conjured by guitarists Hank Shermann and Michael Denner or the monumentally spectral voice of King Diamond himself, this album has something that every fan of heavy metal will love. The superb composition and immersive atmosphere of Don't Break the Oath goes on to influence bands to this very day and makes it totally worthy of this list. 
 
 
 
 

Iron Maiden - Powerslave 
Mercyful Fate - Melissa
Manowar - Into Glory Ride
Sortilège - Sortilège 
Manilla Road - Crystal Logic
Yngwie Malmsteen - Marching Out
Judas Priest - Stained Class
King Diamond - Them 
Satan - Court in the Act
Riot - Thundersteel 

43. Watchtower - Control and Resistance
Easily one of the most underappreciated bands in the metal canon, Texas' prog/thrash metal pioneers debut on the scene with the Meltdown demo in 1984 was a technical genesis on thrash along with the likes of Megadeth's early material and was a part of a movement that would lay the seeds of progressive metal from '85 onward. 
Their sophomore LP, Control and Resistance, had both solidified their importance to this and showed how far they were willing to go even in the face of budding new members that were cropping up in the nascent prog metal and tech thrash scenes at that time. With the inclusion of guitar virtuoso, Ron Jarzombek, and new vocalist, Alan Tecchio, these guys embarked on writing a work that remains a mind-bending, ambitious and brilliantly deranged pillar of the genre it helped spawn. Whether it is on the epics like the title track and The Fall of Reason or the absolute barn-burning shredders like The Eldritch and Dangerous Toy, the composition and performance would still seem relentlessly modern today. 
 
 

I'd say Rob since the JP discog shows more variety out of him than Bruce in terms of range and style. 

8/10 Martin-era Sabbath's pretty great. 
 

Alright, so here are some recs you may enjoy. 
Psychotic Waltz - A Social Grace (1990)
 
Anacrusis - Screams and Whispers (1993) 
 
Thought Industry - Songs for Insects (1992)
Catharsis - Pathways to Wholeness (1995) 
 
Also, all of Alabaster's recommendations are on point with Fates Warning being particularly essential as they had a ridiculous run of quality between The Spectre Within and A Pleasant Shade of Grey. 

I don't like Dream Theater at all, and honestly I'm not the best person for recommendations in this area, but maybe some of this stuff will land with you. You'd probably dig Fates Warning if you haven't heard them yet. My favorite albums are No Exit and Parallels:
 
Also check out their older stuff, I don't like their original vocalist too much but you might enjoy him more than I do:
 
Crimson Glory rules, not very much like DT but highly worthwhile:
 
 
I'd assume you know old Queensryche, but just in case:
 
If you like old Queensryche, you'll probably like Lethal. I guess I wouldn't really call this "prog metal" but it's only a hop, skip, and jump away:
 
Also try Riverside. They're pretty different from what I've mentioned so far, more like modern rock with a bit of a Pink Floyd vibe, but aside from the lame harsh vocals here and there, their first album is pretty cool:
 
On a more metal tip, BAN turned me on to Pagan's Mind at some point last year, again not very similar to DT but I'd be surprised if you didn't like this:
 
And one of my favorite albums from last year is "progressive metal", albeit (again) totally unlike Dream Theater. Howling Sycamore has a hell of lineup (highlight is the original vocalist from Watchtower, yelling his face off) and you might get something out of them:
 
Maybe some of that will appeal. With any luck, @True Belief or @Ecthelion will also swing by and drop some of that sweet sweet knowledge on you.

Iron Maiden - Somewhere in Time 
So much better than Seventh Son. 

Iron Maiden - Somewhere in Time 
So much better than Seventh Son. 

Absolute classic. 

Absolute classic. 

Helstar - Nosferatu
Coroner - Punishment for Decadence
 

Bestial Invasion - Contra Omnes 
Just as good as when it came out little over a year ago. 

If I could narrow down their discography to some essential albums (for me, anyway), it'd look like this:
Images and Words
Awake
Metropolis Pt. II: Scenes from a Memory
Black Clouds and Silver Linings 

Artillery - Fear of Tomorrow

Artillery - Fear of Tomorrow

Anacrusis - Suffering Hour 
Pestilence - Consuming Impulse 
Catharsis - Pathways to Wholeness
 
@Requiem @salmonellapancake I think you guys would like Suffering Hour, if you haven't already heard it. 

Rush - Moving Pictures
Rush - Signals

Anacrusis - Suffering Hour 
Pestilence - Consuming Impulse 
Catharsis - Pathways to Wholeness
 
@Requiem @salmonellapancake I think you guys would like Suffering Hour, if you haven't already heard it. 

Anacrusis - Suffering Hour 
Pestilence - Consuming Impulse 
Catharsis - Pathways to Wholeness
 
@Requiem @salmonellapancake I think you guys would like Suffering Hour, if you haven't already heard it. 

You should check out Sad Legend, if you haven't. 

One of the worst is Velvet Cacoon. 

The problem is that the older material is revered for a variety of different reasons other than simply establishing the genre. Many of the bands operating today would be nothing without the innovations brought forth from bands in the '80s and '90s. That's clearly something worth considering.

Testimonia - s/t
I found out about this band a couple of weeks ago. Italian technical thrash from 1993 and it's really good. It's nice to see that Sadist wasn't entirely alone in doing more progressive endeavors.

Exodus - Pleasures of the Flesh
You know, it's kind of ludicrous how formative this band was to thrash and they basically get ignored overall. Seriously, this album could have just been a recording of Gary Holt's solos and it still would have destroyed Among the Living.