A Comic Reader's Starter Pack

For a "Comic Reader's Starter Pack," I would suggest:

Batman: Year One by Frank Miller;
Sandman: Preludes And Nocturnes by Neil Gaiman;
Watchmen by Alan Moore.

Year One and Preludes are set in very different parts of the post-Crisis DC universe. Preludes mentions the Batman.

The "Watchmen," not called that in the book, are adaptations of the Charlton Comics superheroes who, at the time of Watchmen's publication, were being integrated into the DC multiverse from which they would survive into the streamlined single universe. Watchmen is set in a world where the DC superheroes existed only as fictitious characters and were soon superseded by pirates as superheroes began to appear in the real world.

Year One and Preludes each introduce a series whereas Watchmen, despite later published prequels, is complete in itself. However, the new reader, having been introduced both to comics and to Alan Moore, could follow Watchmen with any other graphic work by Moore - I suggest first his other main superhero series, Marvelman/Miracleman, if this were back in print. In some ways, Marvelman is even better than Watchmen because in it Moore very cleverly adapts an already existent and absurd character to excellent effect. As in Watchmen, Superman was there first but again as a fictitious character.

Next could be Swamp Thing, another series about a super powered being, this one set inside the DC universe - at least the runs by Alan Moore, Rick Veitch and Mark Millar - to be either followed by or read concurrently with Jamie Delano's John Constantine: Hellblazer. Then there is Moore's own work on Superman and his few other DC universe stories. His series featuring the Superman pastiche, Supreme, while relevant, is not among his best works.

Preludes is the first of the ten Sandman collections. These could be followed by Gaiman's several related works, then by his remaining DC universe stories and also by Mike Carey's Lucifer.

Year One is, I suggest, the opening volume of a trilogy which continues with:

The Killing Joke by Alan Moore;
The Dark Knight Returns by Frank Miller.

After that, the new reader could be trusted to find new reading directions for himself!