The Originals: Halloween (1978) & Halloween II (1981) The Lost Sequels: Halloween 4: The Return of Michael Myers (1988), Halloween 5: The Revenge of Michael Myers (1989) & Halloween: The Curse of Michael Myers (1995) Invalidated By: Halloween H20: 20 Years Later (1998)
Halloween is a horror legend, and Halloween II is a generally well-regarded follow up. John Carpenter and Debra Hill, creators of the original and writer/producers of the first sequel, knew that the huge success of their films meant the studio would be clamoring for more. However, they had no interest in extending the story of serial killer Michael Myers and instead devised a plan in which each new annual entry in the franchise would tell an unrelated horror story set on the titular holiday. This was put to the test with 1982's much-maligned Halloween III: Season of the Witch, a movie about cursed killer Halloween masks. That entry's poor reception killed the franchise for six years, but it's not the main subject of this entry - it was never supposed to be in continuity.
In 1988, for the franchise's tenth anniversary, Halloween 4: The Return of Michael Myers was released. It had no involvement from Carpenter and Hill, but is generally considered a solid entry into the franchise and is especially well-liked for the performance of young Danielle Harris as Jamie and the shocking ending. It was followed up one year later by the much more controversial Halloween 5: The Revenge of Michael Myers. Although stylishly-shot and boasting a few very effective suspense sequences, Halloween 5 introduced such ridiculous elements to the series as a crying Michael Myers, an inexplicable psychic link between him and Jamie, a retcon of Myers' modest childhood home into a Victorian mansion, a bizarre cultish symbol, and a mysterious man in black who aids Michael. It has since been admitted by the crew that these elements were thrown in without any thought as to what they actually meant or where they were supposed to lead; the film ended on a cliffhanger that left these completely unexplained.
Frustratingly, it took six years to get any answers - Halloween: The Curse of Michael Myers wasn't released until 1995. Even then, it was a total mess. The script had been written by hardcore series fan Daniel Farrands, but it was constantly being re-written on set during shooting, with producer Paul Freeman making huge uncredited changes to the screenplay. The resulting film was a confusing and hokey mess of occult nonsense that tried far too hard to explain away the oddities of the previous movie. This performed terribly with test groups, and the already-butchered film was sent into rushed re-shoots that were essentially made up as they went along. More blood and gore were added, and the entire third act was reshot to play down the cult angle and intoduce ill-advised, equally hokey sci-fi elements of genetic engineering. The new ending was literally devised on a whim during the last night of shooting, and involved noting more than throwing a couple of props on the floor and filming them in close-up in the hopes the audience would infer some sort of secret meaning.
The movie was critically savaged upon release, and not even the specter of the original Producer's Cut shown to test audiences was enough to salvage its reputation. Just three years later, Jamie Lee Curtis (of the original two films) starred in Halloween H20: 20 Years Later, a return-to-form direct sequel to Halloween II that ignored the characters, themes, and plot points of the previous three films. It was a hit both comercially and critically (at least as far as slasher sequels go), and received its own sequel in 2002 before the franchise was ultimately rebooted entirely in 2007 by Rob Zombie's remake.