Albie, you should check out DT. They're not for everyone - a lot of their stuff is complicated musical wankery. As a musician, I love that - in fact, it's the reason I first got into them. But they also write a lot of great melodic music - in fact, they have a very strong Pink Floyd influence to their sound when they're not playing three thousand notes per second.
If you're into the musical wankery, DT is unsurpassed. All four instrumentalists are world-class virtuosos; there is no other band (not even Rush) with members of that skill level at every position. The ultimate indulgence is the instrumental track "The Dance Of Eternity", but they also write some great songs with complicated fast music. "Beyond This Life" is one of my favorites for that, and "Metropolis Part 1" is perhaps their most legendary song with both great lyrics and complicated instrumental sections.
They're also good with more mainstream metal (even if they make it a little more complicated than usual). "Pull Me Under" is the best known here. My favorite is "In The Name Of God", and "The Glass Prison" is also great for that style.
For the more atmospheric / melodic stuff, "Octavarium" is one of the best (the title song from their 8th album, not the whole album). "Surrounded" is my favorite - not just in this style, but my favorite DT song period. "The Spirit Carries On" and "Trial Of Tears" also excel in this department.
For those with the endurance, the 42-minute "Six Degrees Of Inner Turbulence" combines all of the above and more superbly.
If you're looking for a single album, I'd say the best 4 are:
Images And Words (contains "Metropolis Part 1", "Pull Me Under", "Surrounded", and "Learning To Live" is not to be missed)
Metropolis Part 2: Scenes From A Memory (contains "The Dance Of Eternity", "Beyond This Life", "The Spirit Carries On" - read the Wikipedia article first, as the story is a bit hard to get from the lyrics)
Six Degrees Of Inner Turbulence (title track and "The Glass Prison")
Train Of Thought (contains "In The Name Of God", arguably their heaviest album)