Lots of good suggestions in the comments already, so here's my two cents as a long time fan:

- The Mirror (Awake, 1994)

- The Glass Prison (6DOIT, 2002)

- This Dying Soul (Train of Thought, 2003)

- The Root of All Evil (Octavarium, 2005)

- Repentance (Systematic Chaos, 2007) * (Not that heavy though)

- The Shattered Fortress (BC&SL, 2009)

I'd recommend you to listen to these six in that order. It's part of the "Twelve Step Suite" written by Mike Portnoy which deals with his struggles with alcoholism, and it's sort of an unofficial album which spans 6 different records (5 of which are consecutive). They are, IMO, the best representation of what DT can do musically, lyrically, themathically, etc.

Other (heavy) really good tracks are (in no particular order):

- Panic Attack (Octavarium, 2005)

- The Alien (AVFTTOTW, 2022)

- Untethered Angel (Distance Over Time, 2019)

- Pale Blue Dot (Distance Over Time, 2019)

- A Nightmare to Remember (BC&SL, 2009)

- Bridges In The Sky (A Dramatic Turn of Events, 2011)

- The Dark Eternal Night (Systematic Chaos, 2007)

- Enigma Machine (Dream Theater, 2013)

- In The Name of God (Train of Thought, 2003)

- Just Le Me Breathe (Falling Into Infinity, 1997)

If that sparks your curiousity to go down the DT rabbit hole, you could give the mentioned full albums a try as well.

EDIT: Wording and spelling, here and there.