Great Comic Book Arcs

The Age of Apocalypse arc was very satisfying. It offered a glimpse into a more gruesome alternate universe, one in which the usual heroes weren't always so noble and in which the usual villains were forced into the roles of reluctant heroes. Beast and Cyclops were magnificently depicted as corrupted badguys. Magneto finally displayed his full potential for postive leadership. And the slow, deliberate destruction of Colossus' sanity was fascinating. Throw in some cool, almost ungodly new foes like Post and Holocaust, and you've got perfection.

Also, years ago, there was a Marvel arc in which a group of heavy-hitting villains gathered together and secretly "swapped" foes in the hope of catching each others' adversaries off-guard and unable to formulate a counterstrategy in time.
Red Skull attacked Dr. Strange. Dr. Doom went for the X-Men. Magneto battled the Avengers. Loki blindsided the Fantastic Four; and so on.

I forget the name of this arc, as it was in the 80s, but it was an exciting read for fans of any of the major titles, as people got to see new villains appear and found their heroes in unfamiliar territory. The writing wasn't as crisp or brazen as it is now but for the time, it was quite revolutionary.

A recent arc I liked was the formation of the X-Corps. It wasn't a large arc but it was one of the more thought provoking ideas to come out of the X camp in recent years. What made it interesting was that it gave readers an opportunity to see some favorite bit players in action, both good and bad. Radius, Sunpyre, Multiple Man, Magma, Warpath, Avalanche - they're not always in the spotlight but it was great to see them still in action somewhere, somehow.

Jam it back in, in the dark.