Gomorra didn't do it for me. Very powerful subject matter, but told badly. I'd have to compare it to something like The Wire, which does something similar for the slums of Baltimore, but with far more punch - it has deep painted characters. Gomorrah fails to make you relate with its charactors. When one guy walks away from it all, I don't care. When some people die, I don't care. There's great potential there, but it's not fulfilled. I expect the book is far better.
I'll probably give Watchmen a look unless it gets panned by all and sundry. The book is really quite good. It's not often a graphic novel makes a list like this.