There's been a lot said about whether or not to start a nrew reader off w/ a superhero book. Some think it'd just fuel preconceived notions about the "limitations" of the medium of comics, and turn off a potential reader. Others think if the potential reader is shown something good enough, he'll be converted. I guess I fall into the latter camp. I'd seriously recommend "Daredevil: Born Again" to a new comics reader. I think it's a great treatment of the superhero concept; evena novice would be familiar / the concept of a secret identity, and seeing one get blown and what happens to the hero because of it would probably open their minds to some aspects of comics they may have thought were silly before.
But if you're convinced the reader you're trying to impress wouldn't be caught dead reading a superhero book, I'd give them "Sandman: Preludes & Nocturnes" , just to set the stage (Gaiman was still finding the character, sort of, and this is far from the best of the series, but essential background to get a sense of the character), and give them "A Season of Mists" to read right after that. If the simple, elegant power of that tale doesn't grab any halfway literate person, I don't know what will.
As great as things like "Watchmen", "Powers" and "Kingdom Come" are (and they really are), I agree that what made them important in the genre is their doing away with accepted conventions we had become complacent with as comics fans. They really wouldn't, IMO, have the same impact on a new reader as on someone who'd been into this stuff for awhile. But I wouldnt wait too long to give them this stuff, especially "Watchmen." That's a must-read for any comics fan.