Metamorpho
The Sandman 20, "Facade"
This single issue turned me on to the more outlandish concepts of the DC Universe.
Deadman
Deadman Love After Death
It's possible that this miniseries was the gateway from me reading almost exclusively Vertigo to more Vertigo adjacent DC titles like Grell's Green Arrow and Wagner's Demon.
Hawkman
The Shadow War of Hawkman
This is a great beginner's point for those who want to get acquainted with the character.
Teen Titans
The Terror of Trigon
One of the first examples I had of superheroes as a opera. These five issue made me the world of the Titans seem as rich as the entire DC Universe.
Swamp Thing
Love and Death
For some reason, the initial issues of Moore's Swamp Thing didn't click with me at the time. This story arc captured my attention like few comics have ever done since.
Earth 2
Generations Saga
As someone who grew up in a Post Crisis on Infinite Earths world, the opening arc of Infinity Inc sold me on the charm of Earth 2.