Capaldi is fantastic, but most of his stories rely somewhat heavily on knowledge of earlier episodes, especially in his better seasons (9 and 10).

I’d probably recommend Mummy on the Orient Express as a Capaldi episode for a newcomer, though it is pretty heavy on Clara arc stuff. Listen is another excellent one but, again, a bit Clara arc heavy, and also a little reliant on overall series lore.

I don’t like recommending Blink to people as a first episode because it’s a Doctor-lite episode, and as such doesn’t really represent the show as a whole. But it is an amazing episode.