For Rush, I'd say 1a and 1b are Spirit of Radio and Natural Science.
Dream Theater - A Change of Seasons. The one element that Octavarium is missing is that truly HEAVY guitar riff. Otherwise, they both have everything - key solo, guitar solos, key/guitar unison arpeggios, instrumental wank-fest, tasty bass licks, soaring vocals, and drum fills all over the place - and of course, 20+ minutes.
Led Zeppelin - Gotta lean with Achilles Last Stand