Lightfoot or Stout, the best classes for a halfling in the 2014 rules, and two builds that hide behind their friends.
Under the 2014 rules your race sets your ability score increases, and every halfling gets +2 Dexterity. That covers AC, initiative and attacks for any Dexterity build, from rogues to archers.
The traits are quietly strong. Lucky rerolls every natural 1 on an attack, check or save. Brave gives Advantage against fear, and Halfling Nimbleness lets you move through the space of anything bigger than you. The subrace adds Charisma and better hiding (Lightfoot) or Constitution and poison resistance (Stout).
Lucky does not look like much on paper. Then the halfling rolls a 1 on the save against the dragon's breath, rerolls it, and walks out of the fire. I have watched a Lightfoot rogue hide behind the paladin for six sessions straight. The paladin did not mind.
+2 Dexterity. Your subrace adds +1 Charisma (Lightfoot) or +1 Constitution (Stout).
Advantage on saving throws against being frightened.
You can move through the space of any creature larger than you. Slip past the ogre blocking the corridor.
When you roll a 1 on the d20 for an attack roll, ability check or saving throw, reroll it and use the new roll. It works on every roll, with no limit per day.
The Lightfoot is in the SRD and its traits are below. The Stout comes from the Player's Handbook. The Sword Coast Adventurer's Guide adds the ghostwise halfling, with +1 Wisdom and short-range telepathy; ask your DM before you plan around it.
+1 Charisma, for a bard, warlock or rogue who does the talking.
You can try to hide when you are obscured only by a creature at least one size larger than you. A rogue's Cunning Action lets you hide behind the fighter every single turn.
Mage ArmorMage Armor1st-level AbjurationCastingActionRangeTouchDuration8 hoursYou touch a willing creature who isn't wearing armor, and a protective magical force surrounds it until the spell ends. The target's base AC becomes 13 + its Dexterity modifier. The spell ends if the target dons armor or if you dismiss the spell as an action., but no Intelligence.
Xanathar's Guide to Everything adds feats only some races can take. If your table uses it:
From the Player's Handbook, the Lucky feat pairs well with the Lucky trait, and Sharpshooter suits a halfling archer.
Shoot, then hide behind the nearest ally with Cunning Action. Next turn you attack from hiding with Advantage and Sneak Attack. From level 3, Fast Hands lets you use items as a bonus action.
Build this halfling
Vicious MockeryVicious MockeryEnchantment cantripCastingActionRange60 feetDurationInstantaneousSaveWisdomYou unleash a string of insults laced with subtle enchantments at a creature you can see within range., 
Healing WordHealing Word1st-level EvocationCastingBonus ActionRange60 feetDurationInstantaneousA creature of your choice that you can see within range regains hit points equal to 1d4 + your spellcasting ability modifier. This spell has no effect on undead or constructs., 
Faerie FireFaerie Fire1st-level EvocationConcentrationCastingActionRange60 feetDuration1 minuteSaveDexterityEach object in a 20-foot cube within range is outlined in blue, green, or violet light (your choice). Any creature in the area when the spell is cast is also outlined in light if it fails a dexterity saving throw.16 Dexterity and 16 Charisma at level 1: good AC in leather armor and strong spells. Lucky saves the skill check the party is counting on.
Build this halflingYes, for Dexterity classes. Under the 2014 rules every halfling gets +2 Dexterity, Lucky, Brave and Halfling Nimbleness. Lightfoot is one of the best races for a rogue. There is no Darkvision, and being Small rules out heavy weapons.
Rogue, with a Lightfoot halfling. +2 Dexterity, +1 Charisma and Naturally Stealthy let you hide behind your own allies every turn. Bard is close behind.
Lightfoot for rogues, bards and other Charisma classes. Stout for rangers, fighters and monks who want Constitution and poison resistance.
When you roll a 1 on the d20 for an attack roll, ability check or saving throw, you reroll it and must use the new roll. There is no limit on how often it works.
Build a halfling in a couple of minutes and start an adventure in your browser. Play solo, or send your friends a link and play together.
This guide includes material from the System Reference Document 5.1 by Wizards of the Coast LLC, licensed under the Creative Commons Attribution 4.0 International License. TableForge is not affiliated with Wizards of the Coast. Full licenses.
