Back to the Future I holds up well, although there are a few parts that might be a bit cringeworthy in today's culture. Back to the Future II is still a decent movie but doesn't hold up as well for a variety of reasons, including that any movie that predicates a distant future that has already past and which very little of it came true is destined for issues with the test of time. The storyline in II is also a little bonkers in terms of probability and timing. Back to the Future III holds out OK and remains good fun. Great Scott!
The effects in the Star Wars Prequels did not age well. Way too much CGA, and it's noticeable, especially compared to the far better effects of 7, 8, 9 (and of Rogue 1). The Star Wars original trilogy struggles with sound crispness, graphics, and of course stormtroopers that seem very worthless and comically inept. They are still good movies, but if Disney were to remake them with new graphic and sounds makeovers and edit some of the stormtrooper stuff, it'd probably be worth it. Notably, some of the very worst effects in the original trilogy were the effects added in the late 90s. Some of it was OK, but two egregious and hardly forgivable errors: 1. the added effects for the band in Jabba's Palace. WTF?! Annoying and added nothing. 2. the changed music at the very end of RotJ. The re-done music was lame/boring, and certainly didn't seem celebratory, as was the mood in the original version.
A genre that hasn't gotten any mention yet: the horror genre. Nightmare on Elm Street, in particular, was a very cool concept. In the 80s, the first several were a combination of freaky, scary, and good fun. But oof, the Dream Warriors' graphics were awful. I maintain that the original remains creepy and worth another watch. Other undead scenes with bad 80s graphics: The Golden Child. What about Ghostbusters?