I don't think I do this - the timelines are too narrow for me! Musically speaking, I imagine each 30-year period had at least one kind of event, controversy or 'yardstick' achievement which made it memorable in one way or another. If pushed, I would go for 1911-1940, as this was the period when the 'shackles' from the preceding Romantic periods were being thrown off in earnest by various seismic shifts (Riot at the Rite, the irreverence of Satie and Ives, the apotheosis of Debussian 'impressionism', the 2nd Viennese School, the 'back to the future' ethos of neo-classicism, the first experiments with electronics/sound effects etc) - the problem with choosing this era is that I also love the Romantic music that came before it all!
Yaknow, I often lump Wagner in with late-late-romantic music like late mahler and strauss... he's so ahead of the game, yaknow? its so easy to forget that technically he's more of a mid-romantic than a late-romantic
But yeah. Wagner did it. he completed music. He brought balance to the force. He tossed the ring into mount doom. He saved the village from the bandits. He rescued his niece from the savages. He slayed the dragon and shattered the spear. post-wagner development is just silliness.
(not that i don't really get into that stuff but every once in a while i come back, I remember, Wagner is really all that matters)
(i still haven't recovered from siegfried saturday cut me some slack on these hyperboles)