The Husbands of River Song

Quietly an underrated masterpiece. Genuinely funny, with a fun and engaging story. Capaldi is having so much fun in this and his chemistry with Alex Kingston is probably the best Doctor/River relationship.

As was with Last Christmas and Danny, Husbands serves as a coda for River and it serves to conclude her story in a very bittersweet way, so much so in fact, I'd say it'd be slightly insulting if she ever returned after this - this is the best ending you could give her and I doubt there's anything you could do to justify any return.