The Lord of the Rings. It has become such a big part of my cultural self. I love everything about it, but especially the fact that there is an entire universe Tolkien created, mainly because of his love for languages. That kind of passion is absolutely amazing and it kind of taught me that it's good to be passionate about something. It also taught me that some thing "nerdy" is actually very widely accepted to be one of the greatest works ever written (and one of the greatest films), and in turn it led me to accept that you shouldn't be ashamed of the thing you like, even if they are nerdy/geeky/dumb or stupid in other people's opinion. It also got me super interested into world building, which I'm expressing currently in building a boardgame.
Favourite part is hard, but the Ride of the Rohirrim gives me goosebumps everytime someone even mentiones it.
Two things about getting into LoTR, especially as a younger person, that easily go undervalues:
It is basically a gateway drug to history. The world building goes beyond just "a world", it's a history. In fact I'd argue that Tolkien wasn't aiming at "world building", except for the minimal amount ... but rather "history", "culture" and "geography" building, which is why his "world" feels so real. As a young person, you'll basically become a history nerd without realising it.
It demonstrates very well some powerful ideas about what heroes actually look like. Neither Gandalf nor Legolas nor Aragorn are the heroes of that story. Not even Frodo, as he fails at the end despite his many virtues within the context of the story. It's Sam and Hobbits in general ... the little people with big selfless hearts who made the difference in the battle between good and evil. Eowyn is obviously a relatively feminist figure against the patriarchal backdrop of the world, but without knowing Tolkein's intent with that character, it's a pretty natural character arc when you're already doing the whole Hobbits thing.