Perfect movie franchises are a hard thing to come by. Making a killer debut is hard enough, but to then stick that landing, two, three, or even four times after that is a daunting prospect.
Star Wars has a flawed prequel and sequel series, and The Lord of the Rings has those unfortunate Hobbit follow-ups which reduced poor Ian McKellen to tears during their production. This doesn't take the shine off the legitimately good entries, but it does highlight that, the longer a franchise goes on, the chances of crafting repeat bangers diminish greatly.
However, not every franchise has encountered this problem, with some having managed to maintain a consistent level of quality or just merely avoid the pitfall of a bad release in however many years they've been going for. Critical consensus may disagree in certain cases, and while there's no denying that some of these franchises failed to maintain the same level of quality as their original films, it's up for debate as to whether or not they entered truly "bad" territory.
For this list, there does need to be a slight delineation between "franchise" and "trilogy", as while there have been plenty of stellar movie trilogies, it's unlikely for an iconic movie franchise with four or more entries to have avoided a flawed release. Spinoffs and prequels are also counted here, which, unfortunately for Alien, means it doesn't make the cut.
In any case, the following film series all managed to defy the odds, and deliver the goods over an impressive amount of time.