"Hi, I'm new to comics and interested in what the medium has to offer..."
This statement does not apply to me. But if someone said this to you, and you could give them only one book to make or break their interest in comics, what book would you give them?
I think I would hand them Y the Last Man. To me, the book is perfect, but moreover, it is a fantastical situation, essentially, but not set in the realm of superheroes. Some of my favorite books are superhero books, but for a new reader, I'd want to make sure they knew that comics can be and are so much more.
I know some people who point to Watchmen. For me, I think you get a much richer reading experience from Watchmen if you read after you have some knowledge of comics in general. Not that you have to read all the Charlton books these characters are based on (I never have--but maybe that'd make it even better, who knows), but have more a sense of the flow and poetry of a comics page. Now, I've never read it from the perspective of someone who has never read any comics before, because when I read it, I had already been reading comics for several years. But that's just how it seems to me.
What book would you hand over?