Prepare your own list of Marvel's greatest series and/or arcs. I'm not a seasoned reader, so I really can't do this myself. But, I thought Civil War was above decent.
@OnlyZeusIsFree: Here are some of my favorites:
-Dark Phoenix Saga: a must read for Xmen fans
-Secret Wars: first major Marvel superhero and super villian crossover
-Mutant Massacre: another Xmen classic versus Mr. Sinister and Marauders
-Acts of Vengeance: Classic villian crossover
-Infinity Gauntlet: Thanos with gauntlet = no bueno for everyone else
-X-cutioner's song: some big players....Stryfe, Apocalypse, Mr. Sinister...nuff said
-Age of Apocalypse: epic alternate reality...HIGHLY RECOMMENDED
-World War Hulk: Hulk pretty much whooping everyone else
-Messiah Trilogy: Messiah Complex, Messiah War, Second coming (HIGHLY RECOMMENDED)
-Secret Invasion: Skrulls!!!
-For the cosmic lovers: Annihilation, Annihilation COnquest, War of Kings, Realm of Kings
-Siege: Norman Osborn makes a dumb mistake and tries to invade Asgard