By “Dark Age shakeup,” I mean things like what you see in the following roughly chronological, non-comprehensive list:
-Barry Allen’s death and Wally West’s becoming the Flash
-The Death and Return of Superman
-The Knightfall Saga
-Artemis’ Wonder Woman tenure
-The “Dark Flash” Wallace West storyline
Basically, stuff from the late '80s and '90s that at least temporarily overhauled a major hero’s status quo.
Personally, my favorite is either Death and Return or Knightfall. Death and Return was a smoother reading experience from issue to issue, while Knightfall is more interesting taken as a whole. They both have a ton of padding, though Death and Return frontloads it while Knightfall dumps it all in the middle. I like Wally West and I love COIE, but I actually don’t really enjoy his series pre-Waid. I’m up to the point where he moves to Keystone and it is improving, but that’s taken too long for me to count it as a single plot development.
I haven’t actually read all of these, but the worst that I have read is Emerald Twilight. It’s just mean-spirited, disrespectful, and generally destructive. And I mean, I do have to give it some credit. There’s some genuine pathos in the tragedy of the situation and it does work well as an origin for Kyle Rayner, who I like, but watching a classic hero go nuts and butcher his entire supporting cast is just the worst possible way to make that transition.