My long-time favorite has been Arabia's "Banat Iskandaria" and in particular the medieval version. Mentioned it last time this subject came up, reasons haven't changed. Catchy melody, awesome accompaniment. Familiar with the tune as my teacher made me learn it on my upright bass. Although, it's played at roughly 1/2 bpm for the civ renditions, which really makes it more interesting: the "chill" feel that it has by being so much slower is countered by how deep the dissonances seem to dig in from that timing.
My new favorite, though, is Norway. Can't decide if I like Medieval or Industrial better. The ancient is simply introducing the simple melody. The medieval introduces an interesting cello countermelody (learned that one on bass as well. Think it sounds better on that instrument, though admit I'm highly biased) and a harpsichord-ish accompaniment- those two instruments, cello and harpsichord are two of my favorite instrument timbres in terms of introducing dissonance and resolving it. Then in industrial the cello part being played by a full string section gives it more depth sounding more, appropriately, modern. It's different, wouldn't say better or worse. The accompaniment sounds pretty cool on the harp but gets really awesome when it's covering the melody and accompaniment. The melody is smoothly passed from the harp, then strings, then woodwinds, back to low strings, but every time I hear the low brass come in to complete the phrase I get goosebumps. As for the Atomic era theme, I usually stop paying attention once it starts sounding like Skrillex got a hold of it.
And perhaps it's just me, but this theme creates more imagery than any other theme. When I hear the outtro to the medieval theme, it conjures the vision of a fleet of Viking ships leaving the city after a raid- You may be safe now, but you see Harald standing at the back of the ship and giving you the "Harald's FInger" as if to say, "just because this raid is over doesn't mean we won't be back when you fix everything." And in the industrial version after the brass-gasm, those quick wind stings on the weak beats make me envision a crashing wave, and the hope that the crash of the wave won't reveal the sight of a longship behind it. Finally the way the theme quickly and often shifts from seeming calm and relaxed to intense and grandiose mirrors the shift between peacefully building a civilization's infrastructure to enduring a blitzkrieg Viking raid, over before you know it, and leaving him stronger and you needing to fix everything.