"The ultimate villain of the story, who's causing the problem the heroes must solve."
Note that Big Bad is not a catch-all trope for the biggest and ugliest villain of any given story. The Big Bad is the one who turns out to be behind several other seemingly independent threats.
The Star Wars movie saga has Palpatine (who provides the picture and part of the text), aka Darth Sidious.
Ernst Stavro Blofeld, from the James Bond movies (as well as the books they were based on), up until he was killed in the opening sequence of For Your Eyes Only. Each Bond film also has its own Big Bad:
From Russia with Love: As far as the plot is concerned, it's Rosa Klebb, however, we're made aware of the mysterious "Number One" of SPECTRE who issues orders and terrifies even her.
Goldfinger: Auric Goldfinger. Technically, he's been hired by Communist China, but it's pretty clear that he doesn't care about causing chaos in the west and is just using them to get richer.
Thunderball: Emilio Largo, though "Number One" again appears.
You Only Live Twice: Number One takes direct command this time, and he's revealed to be the aforementioned Blofeld. Again, the Chinese are paying him, but he's the dominant partner by far.
Casino Royale: Le Chiffre is The Heavy, but like other villains in the series he's been hired by third parties (including a Nebulous Evil Organization called Quantum). Perhaps a better example would be the mysterious Mr. White.
Quantum of Solace: Dominic Greene, though other members of Quantum of equal or greater rank than him appear briefly.
Colonel Miles Quaritch, head of the RDA Spec Ops (aka the RDA Security Force) in the James Cameron film Avatar. A corrupt, fascist, and warmongering Space Marine, he is the Enemy to All Living Things on Pandora. Initially, it could be argued that he was in a Big Bad Duumvirate with the sleazy head of the RDA mining site, Parker Selfridge, but by the film's climax it becomes apparent not only that Quaritch has usurped all the power in the base, but also that Selfridge is very much aware of this fact.
Selfridge and Quaritch are a great example of a Non-Action Big Bad and his Dragon-in-Chief. Selfridge is nominally in charge, but Quaritch is much more menacing and is ultimately the true threat.
Push has Henry Carver, and on a more broad scale, the Division itself.
King Ghidorah of the Godzilla franchise fits this trope as he has been Godzilla's greatest rival in six films (one of which he's actually the good guy). He also has faced Mothra in one of her solo movies (and had an expy face her in another solo film), and fought Zone Fighter, a Toho-produced Ultraman-like TV show hero. Godzilla himself could be seen as the Big Bad for some of his appearances.
The Queen alien from Aliens counts for this trope, as it ruins the life of the heroine/lays facehugger eggs throughout the movie.
The Ghostbusters have had to deal with two Big Bads: Gozer the Gozerian in the first movie and Vigo the Carpathian from the second.
Star Trek has Nero, possibly the most pissed off Romulan ever, who is determined to destroy Vulcan and then the rest of the Federation because neither managed to save Romulus from being obliterated by a supernova. He's got a decent reason for being so angry, but good grief, he's got to be the new king of disproportionate response.
Highlander has the Kurgan for the first movie, General Katana for the second, Kane for the third, and Jacob Kell for the fourth.
Marcus Octavius was the Big Bad for the anime version of the story, Highlander: Search For Vengeance.
LordCutlerBeckett in Pirates of the Caribbean. Even though he didn't appear until Dead Man's Chest, you can make a pretty good case for Beckett being Big Bad all along, as he was indirectly responsible for the events of the first movie (branding Jack, initiating the chain of events that made Jack the captain of the Pearl, etc.) despite never appearing or being mentioned, and was pulling everyone's strings in the second. Each movie also has its own individual villain as well:
Captain Barbossa in Curse of the Black Pearl
Davy Jones in Dead Man's Chest, until Beckett manages to take control of him at the end.
Cutler Beckett himself is Big Bad directly in At World's End.
Blackbeard in On Stranger Tides, which is only loosely connected to the previous trilogy.
Sky High has Royal Pain/Gwen Grayson, although the case could also be made for a posthumousBig Bad in the form of Sue Tenny, the original Royal Pain and Gwen's pre-incarnation, who set up the Evil Plan in the backstory before an unfortunate accident involving a weaponized Fountain of Youth set her back a little bit.
The Matrix series has Smith. He starts out as just another Agent who gets more character depth than the others (i.e, he's actively disgusted and angered by people instead of just being an emotionless drone), but after he comes Back from the Dead and starts copying himself, the Machines are forced to reach a truce with Neo to stop him.
The In Unverse books in American Dreamer have Count Renalo, whom Cathy imagines looks like her Jerkass husband. Later it turns out the is a real villain who is a French drug lord.
