This seems like one of those things where there is an obvious answer - friends. After all, what else could you expect from a game that’s inherently rooted in social interaction?
I’ve made a bunch of friends through roleplaying games, or strengthened existing friendships. Roleplaying games have introduced me to new people and have allowed me to see sides of people I’d probably never have seen. So is friendship the thing I’d take away from RPGs?
I think I might have to say something more selfish. While friendship is a component of it, I think it’s the shared creative aspect that’s my favourite non-RPG thing to come out of RPGs. Telling stories, creating things, collaborating together - I’ve been involved in writing fiction with people, creating software and all sorts of other creative endeavors all due to RPGs. Working with people to create things, giving back something to the community of gamers I find myself a part of, that’s my favorite thing to come out of RPGs.
So I guess it’s a close cousin of friendship that’s my favorite - it’s community.