I'm of a mind that, it's only a GN if it was originally printed that way and not serialized.
So as good as Watchmen is, it's a TPB.
It was written to be read a piece at a time. and I think it still works best that way.
So my favorite comic book GN would be Death of Captain Marvel.
And non-comic book would be Wrightsons Frankenstein.
”Comics, more than film, gave us an appreciation for the ways that words and pictures can be used to tell a story… ...Which is why, for us, so many comics today are disappointments. In a medium where you can draw ANYTHING, how can so many artists be content to simply imitate what’s already been done in film and television?”
--The Wachowski Brothers