Surprisingly enough, it's not the movie "Watchmen". That actually ranks a distant fourth, behind "Watchmen: The Motion Comic" in third and "not doing an adaptation of 'Watchmen' in the first place" in second. (Hint: I was not impressed with the movie "Watchmen".)
No, the best adaptation of 'Watchmen' was the first season of "Heroes". I don't think it was deliberate plagarism, but I do think that Jeph Loeb, who was a major player in the first season of NBC's super-hero drama, was heavily influenced by the classic comic book series (he's gushed openly about the book in interviews.) And what he came up with turned out to be a remarkably good treatment of Alan Moore's mini-series.
He ditched a lot of the things that wouldn't work for an audience unfamiliar with the tropes of comic books (the capes, the retired heroes, the code-names...things that worked great on the printed page, but much less well on screen.) Instead, he used a lot of tropes familiar to TV fans--shadowy conspiracies, secret agencies, and rich families with great power and greater ambition. The fundamental concept, though, remains the same--an altruistic, powerful individual who's seemingly withdrawn from the public eye (metaphorically in the case of Ozymandias, literally in the case of Linderman) asks the question: What percentage of the populace is it acceptable to kill in order to bring about a Golden Age? And, having answered the concept to his satisfaction, he proceeds to carry it out.
Of course, as a continuing series, "Heroes" can't actually pull off 'Watchmen's ending of destroying New York. (Although, from what I've heard about subsequent seasons, maybe they should have just ended it like that...) Instead, it cleverly uses prophecy in a variety of forms (Charles Deveaux' prophetic dreams, Isaac's drawings of the future, Hiro's time travel) to create a concrete sense of the consequences in the audience's mind if the heroes don't succeed in stopping Linderman's plans. It also creates a clever ambiguity in the prophecies that it can exploit for dramatic tension--since Sylar and Peter have essentially the same powers, is it going to be Sylar's deliberate act that destroys New York, or Peter's out-of-control abilities?
Ultimately, as with 'Watchmen', all the various interweaving threads come together in a tense showdown, with plenty of dramatic payoffs as the various characters have to make their own moral choices. It ends differently than the comic, of course, but so did the movie. And at least this one didn't have giant blue penises.