H1 - Keep on the Shadowfell

This adventure takes place in and around the town of Winterhaven in the Nentir Vale.

Kalarel, an Orcus worshiping cult leader, is performing a ritual to open a rift into the Shadowfell. Kalarel is served by an elven archer Ninaren who may initially work with the party, and a gnome named Agrid, as well as sundry undead, cultists and other nasties. Kalarel is allied to a gang of kobolds lead by the goblin Irontooth, a gang of goblins lead by their chief Balgron the Fat, and a gang of hobgoblins lead by their un-named chief.

Plot hooks kicking off the adventure include locating a missing person, Douven Staul, an explorer, who has stumbled across the dragon burial ground where Agrid is trying to recover artifacts useful to his master; mapping the Keep on the Shadowfell, built in ancient times to stand guard over a potential gateway to the Shadowfell; and following up a commission to investigate and disrupt the Kalarel's plans.

Allies of the players include the lord of Winterhaven Lord Padraig, a sage Valthrun the Prescient, as well as the ghost of the last lord of the Keep on the Shadowfell Sir Keegan.

The adventure starts with an attack by Kobolds on the players as they travel to Winterhaven, where they are likely to meet Lord Padraig, Valthrun the Perscient and Ninaren. Depending on some players choices they will end up seeking out Douven Staul and fighting Agrid or tracking the Kobolds back to their lair and fighting Irontooth.

Eventually the players will find their way to the keep, or more accurately the dungeon under the ruins of the keep, the first level of which is mainly populated by the Goblin tribe, a Kruthik hive, and some undead guarding Sir Keegan's tomb.

In Sir Keegan's tomb the players can discover the story of the keep - that it was built by the Empire of Nerath to guard against a rift into the Shadowfell being re-opened, but that it was abandoned after he fell into madness and slew his family and retainers.

Additionally in the first level of the dungeon the characters can find a letter linking Kalarel to Chief Krand of the Bloodreavers which ties into H2 - Thunderspire Labyrinth

It is expected that the characters will return to Winterhaven before continuing through the dungeon, and there they discover that Ninaren is working with Kalarel as she has performed a ritual at his instruction that has raised undead from town cemetery.

The second level of the dungeon under the keep sees the characters pitted against the hobgoblins, then undead, and finally they make it to the climactic battle against Kalarel

How should I go about choosing an adventure for a group of new players?

The most important thing is to find something that speaks to you. If the premise leaves you flat, you are not so likely to be able to breathe life into the campaign.

The other most important thing is to find something your players will enjoy. Heck, they'll probably enjoy anything you DM cuz you'll be so great, but if they are all obsessed with dragons, maybe the module with the dragons, right?

What qualities should I look for?

1. Make sure the module you select is for new first level characters. (Unless you really want to start at a higher level, which would make your work a bit harder.)

2. Modules labeled introductory are good for starting DM's and players. They often reprise the rules you will need to be familiar with, and give points to where to look rules, etc.

   CAVEAT: The word "introductory" doesn't have a fixed definition. Some introductory modules will be entirely self-contained, while others might require other materials, such as the Monster Manual and the DM Guide.

3. A module might be (A) specifically for particular game rules (like D&D 5e), (B) be "compatible" with a set of game rules, or (C) it might be just the "story" part without the monster stats. A module of the first type will be easiest to use.

...and how can I tell if a published adventure has them?

I think that a trip to your Friendly Neighborhood Game Store might serve you well. The staff of typically pretty knowledgeable, and you can flip through the merchandise. Other customers often might offer their insights as well.

This site and others have chat forums where you can get suggestions and discuss.

You might also read product reviews online. For the in-depth info you are interested in, I'd favor review website like Escapist Magazine over reviews at online stores (although both have their place).

Should I restrict myself to official WotC adventures on the presumption that they have the best writing and playtesting or is there a way to identify third-party adventures with a similar (or higher) level of polish and quality?

Probably not a bad idea to favor the WotC's D&D 5E material, if that's what you're going to be playing. This has more to do with the encounters being tailor-made for 5E, than one company's material being better than another.

So which are those?

Applying my own answers to your criteria, The Lost Mine of Phandelver in the D&D Starter Kit, and Tyranny of Dragons both would be solid choices. Phandelver also would be a good fit for the number of play sessions you mention.

Tyranny of Dragons is an updated version of the early D&D 5e releases: Horde of the Dragon Queen and its sequel, Rise of Tiamat. Its updates include play balance fixes, perhaps most notably in earliest set of encounters, which make this module more appropriate for beginning DM’s and players.

NOTE: More recently, another official introductory module has been released: Dragon of Icespire Peak. I haven’t read through this one yet, so I cannot comment on its specifics.

You'd mentioned social encounters...

Most D&D modules have a whole lot of fighting involved, and the two I mentioned are no exceptions. Without doing any spoilers, Phandelver probably is more dense with opportunity for social encounters than Horde of the Dragon Queen.

The existence of opportunities for social encounters, of course, has a lot to do with your DM'ing. Monsters don't have to attack the party on sight, even if the module says they do.