A Certified Fresh rating on RottenTomatoes, a respectful box office haul and maybe a couple of Oscar noms to slap on the Blu-Ray case; that's what's needed to make a modern classic, right?
You'd think success in all conventional forms guarantees you, well, success, but audiences are fickle. Opinions aren't some resolute objective fact that remains constant over time. Just as movies somewhat overlooked upon release become regarded as seminal works, so too can a movie lauded upon release become regarded a more than a little trite.
Sometimes it's fair enough - we all got a little caught up in the hype and missed glaring flaws of some wrongly-lauded mess - but just as often it's a case of forgetfulness and anti-hype clouding serious assessment, meaning that movies beloved upon release actually become, if anything, underrated.
Honourable Mention - Avatar
Avatar needs to at the very least be acknowledged when talking about movies where public opinion has flipped - for a brief, three month period there it was the greatest thing since ju ju bees, garnering nine Oscar nominations (it won three) and earning so much money even Star Wars couldn't come close to matching it (in fact, The Force Awakens made only 75% of its total haul). But then, when removing 3D glasses meant you could see the wood from the stereoscopic trees, everything changed.
The movie is now regarded with something approaching derision, its only marked impact on cinema being the proliferation of 3D (yay?). Part of this will be due to people feeling resentment for getting so swept up in the hype, but it (like the reason it doesn't make the list proper) mainly comes down to it being a pretty average film; great to look at and boasting some fine action, but really rather simplistic.