It can't be anything long, has to be something they can reasonably finish. No One Piece, Naruto, Dragon Ball Z. Ideally one season, no longer than two.
It also shouldn't be anything that's still ongoing, has to be something they can literally finish. I'd really like to say Frieren, I think it hits a lot of the right beats to appeal to someone who has never watched anime before, but I wouldn't want to leave them hanging. And we don't even know how long it's going to be in the end, I already said no longer than two seasons anyway.
Has to have a dub. Someone who does not watch anime is not going to read subs. I'm so close to wanting to recommend Apocalypse Hotel as a short and sweet 12 episode masterpiece, but the lack of a dub will be a dealbreaker to your non-weeb friends.
With how much I'm emphasizing that it has to be short, movies would be better than television shows. Which leads us to the extremely obvious answer of Ghibli, anything Ghibli. But maybe Ghibli is too obvious an answer, I know plenty of people who watch Ghibli but have never seen anything else, so maybe we'll say Ghibli doesn't count.
So then I'd say my next answer would be A Silent Voice. Not just because it's my favorite movie, but it's a good example to show that there's so much more to anime than just battle shonen. I imagine most people who don't watch anime may have only ever heard of popular shonen and might not know what else is out there.