One of the best, yet sadly underrated and now extinct, Flash games of all time is Remnants of Skystone. I played it for a year back in its heyday in 2011, and the well-crafted environments, lore, and monsters have left a permanent impression. https://www.youtube.com/watch?v=wyz1cOkbQoQ
A mysterious alien rock had crashed from the sky amongst three warring civilizations, who studied and worshipped this odd artifact. One day, this rock, the "Skystone," exploded, covering the world in noxious haze which spawned freakish fungi and creatures called "mimics." What was left of society took refuge in the sky. Two centuries later, they now seek to reclaim the land, one zone at a time.
The gorgeous art style of this game drew me into becoming a fan of steampunk. Gameplay was also quite impressive too for a Flash game and it was a fully-fleshed-out RPG. Combat was somewhat clunky of course but the original mechanics more than made up for it.
Unfortunately, it was under-appreciated art of its time and the game bled away its playerbase. The developer FlipLine Studios must have been made up of Jews because they left the beautifully crafted world, lore, and characters to rot in order to get shekels off their "Papa's Pizzeria" line of games. They took it off Kongregate in 2014 and now all that's left is the wiki and the memories of a small group of players.