Best storyarc?
Moore’s Anatomy Lesson onward in Swamp-Thing. The best reinvention of a character ever.
The Dark Phoenix saga
The original Galactus story.
Steve Englehart’s Justice Leagues (JLA 139-150). Stunning stuff and completly forgotten.
The Return of Barry Allen storyline in Flash
I could keep going, but…
Worst storyarc? Um…Can I count every “Byrne decides to revamp a book”-storyline?*
Or how 'bout the way Nancy Collins(?) and later writers broke Swamp Thing? Hottest DC book for years and over about 20 issues, they ruined the character so badly that we’ve only seen Swampy about 15 times since his book ended about 10 years ago (the recent mini-series about his daughter notwithstanding)?
Or Gerry Conway’s Justice League with Vibe, Gypsy, Vixen and so on?
The Spider-Clone, of course. (Note to writers: don’t start a mystery until AFTER you’ve worked out whodunnit and why)
The Green Lantern/Green Arrows by O’Neil and Adams (IMHO, of course: I realized that these are loved by most people. But the characterization is just plain wrong (Hal’s not a snivelling wimp**), they’re preachy, the dialogue is terrible etc)
The three part “Emerald Twilight” crap. Like Kyle Raynor or not, “Emerald Twilight” makes no sense (to the extent that it does) unless you ignore the previous 20 issues (and they did).
[sub]I’ll stop now[/sub]
Fenris
*Note: I don’t dislike Byrne’s stuff, as long as he’s not breaking other people’s toys. And again, I highly recommend his current Elseworlds series for DC: Generations
**Allegedly, one of the reasons that Emerald Twilight happened (beyond the obvious: the Death of Superman and Breaking of Batman series sold really, REALLY well) was that the powers that be (including O’Neil) were upset that Gerard Jones, the writer prior to Emerald Twilight had done a short story arc explaining that Hal’s behavior in the O’Neil/Adams stuff was due to psychic brain-damage (Bwah-hahaha… I loved it…) and kicked Jones off the book.