Studios have been producing comic book movies for decades, but not until the end of the 1980s did they begin to become any good. Only since then have they begun to strike an appropriate balance among visual interest and polish, faithfulness to the source material (to please the existing comic book fan-base), effective transfer across media and accessibility to outside audiences (those other than the existing comic book fan-base). Amid the proliferation of comic book movies since then, some few stand out as the best the genre has yet offered.
One of the keystones of the comic book movie has to be the 1989 "Batman," starring Michael Keaton and Jack Nicholson. What IMDb says of the movie can be easily found, but what is important for this discussion is that the film served as proof that comic book movies could be popular for broad audiences. "Batman" makes it easy; the visuals were remarkably polished for the time, it involves the most iconic characters in the comic book franchise and has them played by actors who fit the parts well, exquisite music (a key feature of films which comics themselves lack) from the pen of Danny Elfman and an easy-to-follow plot that eased audiences into the universe of Gotham City and its Dark Knight. Without it, the success of the Christian Bale Batman films--"Batman Begins," "The Dark Knight" and "The Dark Knight Rises"--would not likely have been possible (although the world might also have been spared such things as "Batman Returns").
Another of the major comic book movies is the 2000 "X-Men," starring Sir Patrick Stewart and Sir Ian McKellan. The film is remarkably pretty, doing well at translating the (jokingly referenced) bright spandex and thinly-veiled social commentaries of the comic page into the darker, more "realistic" film, complete with thinly-veiled social commentary, much of the mainstream film-going audience of the early twenty-first century wants. It does deviate from some of the comic continuum in its depiction of character origins and arrangements, but it makes up for doing so in capturing the essences of the characters as they developed on the printed page, including their key conflicts. It also set the groundwork for a semi-independent series of X-Men films, including two direct sequels, prequels including "X-Men: First Class" and the Wolverine films, and the upcoming "Days of Future Past." As the binding of the series, however, and the entry-point for the general audience the 2000 "X-Men" has to be considered most important of the films.
Like "X-Men" in being the lynchpin of semi-independent series of films is 2012's "The Avengers." Its visuals borrow from its several prequel movies (two Iron Man films, the 2011 "Thor," "Captain America: The First Avenger" and the Ed Norton "Hulk"), grounding it firmly in the established film continuum. The visuals, including the excellent casting choice of Scarlet Johansson as Black Widow, do much to draw in a general movie audience; the film is well worth looking at, from its dramatized heroes to the humanized supporting characters to the intricately-rendered opponents. There are some deviations from comic book canon in the film (notably the absence of one or two of the more notable Avengers), but they are more than made up for by the pacing of the film, social commentary (including a negotiation of generation gaps) and witticisms so frequently sought by the movie-going population. And, as the lynchpin of several series (with more sequels on the way and seeming to grow organically from it, as in the case of the 2013 "Iron Man 3"), "The Avengers" stands out as a singularly important piece of comic-book cinema.
Other movies can be argued to be among the greats of comic book film, but these serve as useful benchmarks for comparison, both for the films already released and those yet to come.