I picked Harry Potter. A solid argument could be made for it being the best YA novel adaptation. I read Lord of the Rings as a child but I'm not sure if I'd consider it a young adult novel. It also has an argument for being one of the most faithful book adaptations, especially for a series. I also think HP had a ton of influence not only for its generation of viewers but for the film industry in general. The wave of YA novel adaptations that followed were a direct result of HP's success. There is no Twilight, Hunger Games, Narnia, etc movies if HP doesn't kill the box office like it did. Some of y'all will say that'd be a good thing, but influence is influence none the less.
Terminator trumps HP in the influence department, though. It's a benchmark series in the action/sci-fi genre and visually ahead of it's time (aka a James Cameron movie). The only classics in this debate are in the Terminator franchise. The most important movies in regards to cinema history are in the Terminator franchise. But the amount of just straight up bad movies hurts it in this comparison. Unlike the franchise I would compare it the most to(Alien) I haven't been impressed, or even interested, in any of the entries in the series in the last decade.
I literally finally watched the entire Harry Potter series earlier this year on HBOGo. I was in elementary school when the books first came out, and in college when the 7th one dropped. I bought the first two movies on DVD(
)but couldn't be bothered to follow the series after because the books were all I cared about. So I was surprised to approve of the series as much as I did watching it so long after the fact. That said, along with Jurassic Park, Terminator and T2 are some of the earliest movie experiences I can remember and what made me fall in love with movies in general. I'm very confident that any kids who had the HP movies be their first memories are way more happy with the direction that franchise took than those of us who remember the beginnings of the Terminator franchise