Now, I have some rules here. There are only "superhero" comics represented, and each hero is allowed no more than two spots on this list. Other than that, anything goes. The titles of the books will be links to their Amazon pages. Some of the entries contain spoilers.
#10 - Civil War (originally published in 2006-2007) If I'm being completely honest, the only reason that this is on here is because people that usually don't give a shit about comics made such a big deal about it, and it made the Marvel universe what it is today. The first four issues of this seven-part comics event were the best; then it simply fell apart. The heroes were divided after a one team caused the deaths of thousands that the U.S. government demanded they register their real names with them. Iron Man backed the government, and Captain America (shockingly) backed the rebellion. This had so much potential to be one of the most epic things to ever happen in comics history, and it failed. Miserably. Even worse, Marvel's recent attempts at trying to pick up the pieces (with Spider-Man's "Brand New Day," and the like) have been greatly disappointing. Read this knowing full well that it may have ruined Marvel's once-perfect reputation for the next few years. Such a pity.
#9 - Iron Man: Demon in a Bottle (originally published in 1979)The reason this story is still relevant today is because it showed that heroes weren't only weak to villains. It showed an "average Joe" side of Iron Man, AKA Tony Stark, who weathers a shitstorm like no other in the first few pages of this book, and tries to wash it all away with alcohol. It made the character seem real and multi-dimensional, something that hadn't really been done in comics before. Even better, his once-rampant alcoholism effects his character today. This is also rumored to be the basis for 2010's Iron Man 2.
#8 - Spider-Man: Kraven's Last Hunt (originally published in 1987)I have a soft spot for Kraven, which is why this is on here. Though he is the Steve Irwin of the Marvel universe, I still love him. Frustrated with his fruitless attempts to capture and/or kill Spider-Man, Kraven begins going insane, and hatches a scheme that ends up putting Spider-Man in the grave. After doing so, he goes even more insane, donning Spider-Man's costume and catching criminals (including the "uncatchable" Vermin). The real Spider-Man ends up escaping from his tranquilizer-induced dirt nap and confronting Kraven, who ends his own life because he has nothing left to hunt (or prove). This story was one that made it okay for comics to have themes of suicide and insanity. This book broke new ground, not unlike Demon in a Bottle.
#7 - Marvel Zombies (originally published in 2005-2006)Ah yes, Marvel Zombies. A good idea that was made better by fantastic writing, interesting art, amazing covers (based on famous Marvel covers from the past), and an absolutely fucked up "friendship" (between Ant-Man and Black Panther). The premise is pretty straightforward: most superheroes in the world have been infected with a virus that makes them hunger for human flesh, all while retaining their personalities and powers. It just goes to show you that alternate universes are awesome, when done right.
If you like this, check out all of the others: Marvel Zombies: Dead Days (the prequel), Marvel Zombies 2 (the sequel), Marvel Zombies VS. Army of Darkness (amazing crossover), and the recently-published Marvel Zombies 3 (which I haven't read yet, surprisingly).
#6 - Batman: The Long Halloween (originally published in 1996-1997)When you have a team like Jeph Loeb and Tim Sale writing and drawing your comic event, you know it'll be good. This 13-part series was essentially a murder mystery, starring a new villain named "Holiday," who kills seemingly unrelated people on...holidays. It also shows how Harvey Dent became Two-Face, something that hadn't really been touched on before. It also stars many of Batman's villains, including the Joker, Scarecrow, and Catwoman. It also shaped Gotham police commissioner Jim Gordon into the character he is today. The ending is absolutely amazing (almost poetic and beautiful), and has sparked many, many debates and theories amongst comics fans. Parts of this book were unsurprisingly taken to make this summer's best movie, The Dark Knight.
If you like this, read the sequel: Batman: Dark Victory. It's almost as good (but not quite). Also, read the Alan Moore-written Batman: The Killing Joke. It also inspired the movie.
Tomorrow, I'll be posting the top five. Get excited!