Jonathanjong, for Dream Theater, I really consider them more of a jazz/metal combination (maybe I just don't recognize their classical influences though, since I don't listen to classical).
My favorite album by them would be either 6 Degrees of Inner Turbulence, Scenes From A Memory, or Images & Words. If I HAD to recommend one, I'd probably recommend Six Degrees, although I've heard it's not exactly a fan favorite.
For good individual songs, check out Space-Dye Vest, Pull Me Under, Take The Time, Metropolis Pt. 1, Overture 1928, About To Crash, Solitary Shell, About To Crash (Reprise), and Losing Time / Grand Finale. My favorite of these songs would probably be Metropolis Pt. 1.
Oh yeah and two (supposedly) epic songs by them are A Change of Seasons and Octavarium. I've never heard A Change of Seasons, but I find Octavarium boring tbh. It does have this ridiculous verse where pretty much a ton of allusions to their musical influences are used.
Check out their offshoot project, Liquid Tension Experiment. That's more influenced by jazz.
I have Quantum by Symphony X and find it sort of boring.
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Reponse to your post in the Shakira topic, figured you'd have a better chance of noticing it here.