1) JLA: New World Order
This is what brought me fully into the DCU.  Before, it was a big giant meh.  It directly addressed everything that I had cluelessly assumed was wrong with the DCU: Batman is useless.  Threats not big enough for the heroes.  Lifeless storylines.  
2) Green Lantern: Mosiac
I'm a huge John Stewart hog, and this is about as good as it gets for me.  It's so trippy and touched on African American stuff and he was still a Superhero at the end of the day.  Ownage.
3) Teen Titans: A League of Their Own
Comedy.  Adventure.  It hasn't been rivaled yet, and there's nothing that beats that combination for me.
I don't really need anything else.  Except that first Authority TPB, but I don't have it, and forget what it's called.