A surprisingly excellent adventure. I've used it on multiple occasions to introduce players to 5E, including 3E players, 4E players, and total newbs.Beginners set - Lost Mine of Phandelver
Assuming you don't want to run this one, I'd suggest something simple, such as Keep on the Borderlands (which was partially reprinted for the playtest, if you can find it), Sinister Secret of Saltmarsh (1E), or the Secret of Bone Hill (1E). Death House (from Curse of Strahd) is also pretty good for a one-shot. Ghost of Dragonspear Castle (from the end of the playtest) is pretty good, but requires jumping levels to complete (4 adventures take characters from level 1-10).