Lately, the Millennium series seems to be taking over this role, due to a lack of continuity and a perceived overuse of tropes lifted from popular anime of the time, though there is still some contention within the fandom on this.
For a long time, the Timothy Dalton movies were seen as a Dork Age, with Licence to Kill being so gory and violent that many felt it barely resembled a Bond film. Nowadays, however, the Dalton movies are seen as prototypes for the Daniel Craig era, having had the bad luck of hitting about twenty years too early. In addition, the Bond that Dalton portrays is much closer to the Bond that Ian Fleming wrote: a stone-cold killer with a hinted-at lust for violence whose womanizing, used to paint him as a playa during the Connery era, made him come across as a sexual predator.
The consensus on the Pierce Brosnan age is that it got progressively worse as time went on. Golden Eye, the first film with Brosnan as Bond, is the only one with a Fresh score on Rotten Tomatoes (even Moonraker has a Fresh score on RT, although just barely), and is Brosnan's equivalent of The Spy Who Loved Me or For Your Eyes Only as the film that even his critics tend to enjoy. (It also had an absolutely kick-ass video game adaptation, which makes it nostalgic for a whole generation of '90s kids.) On the other hand, it ended with Die Another Day, which fan consensus views as a rival to Moonraker as the worst film in the series.
Even Brosnan doesn't seem to think fondly of his tenure as James Bond. In a documentary made to commemorate the 50th anniversary of the franchise, Brosnan said he can only remember filming Goldeneye. The rest "blurred together."
A large chunk of the fanbase was expecting this when Daniel Craig was announced (there was a "Bond's Not Blond" movement after his announcement), but was averted when shown that Craig was actually pretty awesome; like Dalton, his films hewed much closer to Bond's characterization in the novels, acting as an origin trilogy for him. That said, some Moore and Brosnan fans consider the Craig era to be a Dork Age, arguing that Craig-era Bond has taken so many pages out of Jason Bourne's book that he no longer resembles Bond at all. Specifically, the plots are heavily toned down from past films, with few of their over-the-top villains, gadgets, or science fiction elements. Within the Craig series, Quantum of Solace is usually held to be the worst of the three, thanks to poor direction and a plot that tried to marry the Darker and Edgier style of Casino Royale with a more conventional Bond Super Villain, with mixed results (stealing Bolivia's water just doesn't measure up to past Evil Plans).
Mae West lost a good chunk of her sex appeal when The Hays Code was imposed, but her movies remained passable. Myra Breckinridge and Sextette, made after she was convinced to come out of retirement in old age, are not. Myra Breckinridge was a terrible film in its own right and only featured Mae in a single scene — basically playing herself — as a man-hungry talent agent-type who gives the eponymous hero/heroine (don't ask) lessons on mistreating the menfolk. Frankly, Mae is not the grossest thing in it — not after you see the strap-on scene. Sextette... well, it was based on her 1926 play Sex, and having her be a Memetic Sex Goddess back when she was 32 was quite a different matter from having the movie treat her like one when she's a frail, overly made-up 84-year-old woman paired with men young enough to be her grandchildren (such as 32-year-old Timothy Dalton, for whom this is a major Old Shame). Most people's reactions to the film are somewhere between a Primal Scene reaction and profound Squick.
Depending on your opinion, either Alien³ or Alien: Resurrection. While in popular culture, Alien 3 is considered the turning point of the franchise, many fans of the franchise appreciate the Assembly Cut's character drama (with an Alien...) approach, leading some to believe that the latter example is the true turning point. To support its quasi-popularity, many fans saw Alien 3 as a return to the themes and atmosphere of the first film, where the second was a subverted gung-ho action flick.
The Alien vs. Predator films are considered the nadir of bothfranchises. While the two have crossed over before, the films were considered inferior to previous team-ups and both films have the two lowest Rotten Tomatoes scores of all the films. note 21% & 12% for the first and second film respectively. While Alien vs. Predator was criticized for its watered-down PG-13 rating, Aliens vs. Predator: Requiem was panned for trying too hard to go in the opposite direction. Both the Aliens and the Predators underwent severe Villain Decay and later films have rendered the spinoffs non-canon.
Some argue the Marx Brothers went through this after their switch to MGM. Zeppo got tired of acting and the studio forced the brothers to go from completely anarchic Rapid-Fire Comedy to more good-natured characters helping out a forgettable romantic lead between increasingly tedious musical numbers (Groucho called The Big Store's "Tenement Symphony" "the most godawful thing I'd ever heard"). Granted, there was still plenty of CMOF, it was just more restrained than during their years at Paramount.
Bowfinger is another exception to his mid-career slump. It is generally considered some of the best material to come from both Murphy and Steve Martin in years. It was a box office success and currently holds an 80% on RT.
Then there's the straight-to-video sequels. Well, as always YMMV, but generally when Hellraiser fans recommend the series to someone they're usually only talking about the first two films - and maybe, possiblyBloodline.