It's more of a long-form arc, with multiple arcs taking place in between. I classify it as an arc because it starts with Zoom's origin and his attack on Linda, and at the very end, Wally closing the loop and setting things right. Continuous threads moving from start to finish.
X-Men: Inferno
Though it starts way back in Uncanny X-Men 168 with the introduction of Madelyne Pryor, Inferno is the culmination of a buildup not unlike The Dark Phoenix saga had. This is back when X-Men writers were allowed to plan several months or even years ahead, and it shows. From Uncanny and X-Factor all the way to the final showdown with Sinister, I think it's a strong contender for being some of Claremont's best work on X-Men.
Superman: The Journey
Not many people's favourite arc, but for me it's easily my favourite. We have a Superman that, as powerful as he is (and he is written as powerful), is forced to confront the fact that as much as he might have worked to earn the admiration and trust of all around him, that it could so easily be taken away from him. We see him mess up, and how easily people turn on him. For anyone that doesn't know this arc, it's the arc taking place from Superman's POV leading up to the Sacrifice fight with Wonder Woman.
Aquaman by Peter David
Yes, all of it. I don't care. I could just say The Atlantis Chronicles. Or Time and Tide, but in all honesty, those plus the first 47 issues of Aquaman v5. It's brilliant. World-building. Smart-writing that has nuanced, interesting characters. Everything positive people say about David's run on the likes of X-factor and Hulk is just as much true of his run on Aquaman.
Truth: Before I read it, I had zero interest in the character. Zero.
Final Crisis: Superman Beyond
An arc that, imo, has aged incredibly well in light of watching the likes of Snyder and Johns attempt to emulate Morrison's skill at crafting a story on this scale. And that's not even me saying that what they did was bad. This story is just amazing. And it takes place over the course of two issues. TWO. It deals with themes that make you go back and read just to try to grasp the damn thing, and at the very centre of it all, at the very core of the story, is literally a guy that just wants to save his wife. It's arguably Morrison's best work, for me, and better than most Superman stories, it balances the "super" and the "man" incredibly well.
Yeah, tbh that Flash run is part of why for years Wally was arguably my favourite DC character. It's also part of why I tend to defend Johns even when he's not so great.
And honestly, I was trying to keep it to one character/team, and one writer per team. When it got to the fifth one, I was stuck asking myself if I had really enjoyed Hush or Knightfall more than Superman Beyond. Or if The Dark Phoenix Saga was separate enough from Inferno to really count. I could have put Hulk by Peter David, but that would be Peter David twice etc.