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.