28: Thing you’d be most surprised a friend had not seen or read?
Star Wars has both the lowest bar of any geek media universe (one movie, and as of December you have two basic options for which one) and the highest profile, not coincidentally. The radio series about celebrities doing things they’ve missed or avoided before is called I’ve Never Seen Star Wars for a reason.
For reading, I guess The Hobbit. Lord Of The Rings takes time and effort - The Hobbit takes a couple hours. (Hence the joke that if you don’t have time to watch one of the films, just read the book.)
Not played would seem more relevant than not seen or read. So probably D&D, because it’s omnipresent. Even though its omnipresence and age made me personally avoid it for decades, up until 3rd edition, I still knew it well enough that I could have played it (at least as a Fighter) from about age twelve.