10 buy or die/essential/if you don't have them you suck, progressive metal albums for me:
Queensryche - Rage For Order
What is probably the actual beginning of the genre, still sounds more progressive and metal than the vast majority of prog metal albums out there.
Watchtower - Energetic Disassembly
THE FIRST album that could be called "progressive metal". It is thrash above everything else, but it marked the first time that the music of Rush was incorporated in metal so obviously. Cartoonish melodies and bizarre song structures, overall the definition of "ahead of its time". Of course all metal other bands that tried to be so much technical were always compared with this one. And lose the game.
Psychotic Waltz - Into The Everflow
Simply one of the most original, innovative and unique metal albums of all time. Trippy and psychedelic in some parts, still 100% metal. Lots of Pink Floyd influences in the most atmospheric moments.
Fates Warning - Perfect Symmetry
The absolute, not my favourite, F.W. album, the title says it all. Still unreachable by almost all other bands of the genre.
Sieges Even - A Sense Of Change
For some period of time i just called this "perfect music". And it is. This one is one of the most emotional pieces of prog ever written. Not as much "metal" as the other albums on this list, but it doesn't matter, if you catch it's feeling, you'll be in love with it forever.
Shadow Gallery - Tyranny
Since you wanted something similar with Symphony X, these guys will totally make you fart your tears. Seemingly complex, but very direct on what it wants to transcend to the listener.
Vauxdvhil - To Dimension Logic
Another prog metal album that just doesn't sound like ANYTHING else, again the focus is given in the atmosphere that is dark and kinda space-y sometimes.
Heir Apparent - One Small Voice
Completely ahead of its time, using keyboards with traditional metal riffing like no other band has done before. Also Steve Benito (the singer) has one of the most incredible voices ever in metal, his high pitched vocals will make you piss your pants.
Eternity X - The Edge
One of the most perfect concept albums about a sane man slowly falling into madness + one of the most intense singing in the genre, especially when the singer does the voice of the character's lunacy, love it or hate it sometimes. Highly exceptional melodies.
Soul Cages - Moments
The best band influenced by mid period Fates Warning, 2 singers, one male, one female, usually singing with their voices combined creating a fantastic result. Tons of inspiration and emotion in this one too.
For obvious reasons Dream Theater's "Images And Words" was excluded.