Sylas has been a literal wrecking ball when it comes to Demacian lore and the events within Demacia itself. While it has been spread across a few months since his release, Sylas has:
* Made Lux come clean that she has magical ability to her brother and family.
* Either killed J3 or (more likely) was the crux to force another party to kill J3
* Has had J4 assume the throne so he is no longer prince.
* Plunged Demacia into Martial Law/Civil War.
* Mageseekers. Just mageseekers dude. You can't trust them.
* Has given us a look into Xin Zhao's aftermath and relationship with J3.
* Introduced us to the extended (and possibly very Lawful Neutral/Evil) Crownguard family with Tianna and her husband.
By all accounts I can't think of a single champion who has spurred this much change and character growth in a region. The only event that can possibly come close is Gangplank's rework but even then, the scale of the changes was on a much smaller scale as Bilgewater is a "smaller" region overall. Not to mention the story in Burning Tides was a little shorter and smaller than Sylas.
I'm usually critical of Riot's inability to follow-up stories but they've certainly made Demacia as a nation progress since Sylas's introduction.
EDIT: Azir is brought up a lot but I disagree with Azir. Azir's introduction wasn't a story progression of Shurima but rather resetting the story of it/retconning what occurred. A few characters _did_ get progression (Mostly Sivir and Cass) but most of this was just setting up Shurima to be an actual nation rather than a loose collection of psuedo-egyptian ideas. Conversely, Sylas doesn't retcon or change any history but rather takes the story of the region and makes it continue in a meaningful and interesting way.
Azir is a retcon and rewrite. Sylas is Chapter 2.