To be sure, some of my "top 20" won't be recognized as films with superheroes in them, though I certainly could justify the featured heroes as belonging to what I call the "superhero idiom."
MARK OF ZORRO, THE (1920)-- The first cinematic adaptation of Zorro, as well as the film that elevated Douglas Fairbanks Sr to adventure superstardom
SEIGFRIED (1924)-- Fritz Lang's ambitious adaptation of the Seigfried legend
MASK OF FU MANCHU, THE (1932)-- *Not* the first film adaptation of classic "super-villain" Fu Manchu but still an over-the-top delirious delight
TARZAN THE APE MAN (1932)-- The first Johnny Weismuller Tarzan, and still the definitive jungle-hero.
ADVENTURES OF CAPTAIN MARVEL (1941)-- The best chapter-serial adaptation of a comic-book superhero.
7TH VOYAGE OF SINBAD, THE (1958)-- The best venture by Ray Harryhausen pitting heroic swains against stop-motion monsters
GOLDFINGER (1964)-- The best of the '60s superspy films.
DIABOLICAL AXE, THE (1965)-- Mexico made dozens of wrestler-superhero films, but this one, which gives Santo an origin uncannily like that of Captain Marvel, is excellent escapism.
THE FIRST STAR WARS TRILOGY (1977/1980/1983)-- Well, it really is one long continuity spread out over three films, so I'm counting it as one big film.
SUPERMAN II (1980)-- Once the origin was outta the way, the sequel was free to be a little less portentous.
RAIDERS OF THE LOST ARK (1980)-- Valentine to the adventure-serials of yore, albeit with a supernatural angle they usually didn't attempt.
CONAN THE BARBARIAN (1982)-- Not always true to Robert E. Howard, but closer than any other live-action or animated version.
ROBOCOP (1987)-- Best of the "future-Earth" superheroes
BATMAN (1989)-- Whether you think modern superhero movies are a good thing or not, Tim Burton gets the credit for making them viable as adult entertainment.
HEROIC TRIO, THE (1993)-- Hong Kong's best take on the superhero genre
CROUCHING TIGER, HIDDEN DRAGON (2003)-- Wire-fu *wuxia* superheroics.
X2: XMEN UNITED (2003)-- Another #2 sequel that outdoes the original.
SPIDER-MAN 2 (2004)-- And yet another #2 that outdoes #1.
IRON MAN (2008)-- Robert Downey, jacked up on armor.
INCREDIBLE HULK, THE (2008)-- Still not the absolute best take on the Hulk, but the closest so far.
It'll be interesting to see if any of 2011's offerings force me to drop one or more favorites. GREEN LANTERN certainly seems a contender-- but I thought that about THOR, too.