I've actually never thought
Watchmen was really all that influential or anything by itself, it's simply the best single work for a lot of things that were happening at the time in comics. It probably had more influence as a successful mini-series at the same time as
Crisis and
Secret Wars. Nobody really copied the frame story, or the panel setups, unless they were doing deliberate call backs. All the "realism" of superheroes was being done at the same time due to
Crisis, they couldn't be influenced by a book coming out at the same time.
Moore's
Swamp Thing is arguably way more influential in selling first, the entire platform of Vertigo (which birthed
Watchmen), but then second, all kinds of reinventions of classic characters and especially the reinvention of the superhero as another genre book.
Also
Supreme is his best work probably.

The movie making V into a slightly left-wing Democrat who is raging against the Bush Administration was the one time Moore was right to be afraid. He's been wrong in like every other instance of people trampling on his works, people go out of their way to stay true to Moore more than almost any other writer who never gets that treatment.