SOURCE: Player’s Handbook

Sentinel’s Lovely Array of Benefits

Benefit #1 – 

When you hit a creature with an opportunity attack, the creature’s speed becomes 0 for the rest of the turn.

This is a remarkably good way of locking creatures down; opportunity attacks are normally a tax someone pays for escaping, but now they’re simply unable to do anything. A very solid benefit.

Benefit #2 – 

Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach.

The usefulness of this depends on how often your DM uses the disengage action. Some DMs will just have monsters disengage, while others want them to take other actions before or after leaving. Some DMs might even vary it depending on the monster.

Benefit #3 – 

When a creature makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.

Depending on ally positioning and monster tactics, this could be anything from useless to a free extra attack every round. On average a decent benefit, but your mileage may vary.

Mechanics and Requirements

The Intricacies of Sentinel

Remember that opportunity attacks are a reaction and the same with the special melee weapon reaction attack. You can’t make both the opportunity attack and the other reaction attack since you only have one reaction, and you especially can’t combine it with all the other reaction features in the game like the Shield spell.

The free melee attack from benefit #3 is a normal melee weapon attack (not a booming blade cantrip attack) made with whatever you’re currently holding: so you had better be holding a melee weapon. There are, however, ways to make the opportunity attack into a cantrip or to enhance it further: more on that later.

Remember that your character completely ignores the effects of enemy disengage actions, but not anything else that prevents opportunity attacks like being unseen or just teleporting away.

Finally, note that you get the special reaction attack when the enemy’s attack is aimed at a target other than you; it doesn’t have to be a creature. This can synergize with things like the Mirror Image spell, for example.

For more information on opportunity attacks and Sentinel, Jeremy Crawford clarifies the intent behind them here.

Key Stats

This is one of the feats that does not come with an ASI, but thankfully the feat is powerful enough to where this isn’t a big deal. However, it does rely on you landing your hits, so whatever stat you make melee weapon attacks with (usually strength for melee characters, but occasionally dexterity or even some more exotic stat like charisma) is vital for this feat. You don’t want to take this feat with a mere +1 or +2 in your attacking stat.

Ideal Characters for Sentinel

Top Classes

Paladin – Paladins can benefit especially well from Sentinel because their various class and subclass aura features incentivize them to stand near allies anyway. When the enemy decides to attack the squishy rogue or wizard instead of you, you can punish them with a reaction attack, which could have a smite attached. You can also lock down enemies if keeping them away from your allies is more your style.

Rogue – Rogues have a phenomenal reaction attack since they can sneak attack once per turn instead of once per round. If you can somehow make the enemy target another ally instead of you—perhaps someone even squishier—then you can get two sneak attacks in a round. The Arcane Trickster can use Mirror Image to get this reaction sneak attack as well since the mirror images count as different targets than you.

Artificer – The Battle Smith artificer is a good subclass, but enemies may target the steel defender instead of you like you probably want them to; your armor class is vastly higher, after all. Take this feat and punish people for attacking your trusty robot friend.

Multiclassing Considerations

Sentinel doesn’t particularly benefit from multiclassing, but it’s not inhibited by multiclassing either. You can take it on any straight classed or multiclassed character and expect to get some solid benefits out of it.

Race or Subrace Choices

No races particularly synergize with this; things like the Bugbear can’t get their 2d6 on the opportunity attack or special reaction attack since this feat is entirely based around reacting to what enemies do, but keep in mind that small races have disadvantage with heavy weapons, which tend to be the best weapons for making opportunity attacks with.

Combos, Tactics, and Synergies

Complementary Feats

If you’re going to get more attacks, you might as well enhance those attacks as much as you can.

War Caster lets you replace opportunity attacks with a cantrip such as Booming Blade, which can be useful on some builds.

Polearm Master lets you do a devious combo; creatures provoke opportunity attacks when they enter your reach, and your opportunity attacks freeze people in place, so you can get a polearm with a ten-foot reach and lock people ten feet away from you, assuming you hit. If they only have a five-foot reach themselves, they’re screwed.

Great Weapon Master lowers your chance to hit, but it usually increases your overall damage, and if you often get advantage on your attacks, it dramatically increases your overall damage. Dealing 1d10+15 instead of 1d10+5 on a reaction attack is good.

Spells that Synergize

Anything that provides another target like a summoning spell is useful, so long as the enemy targets the summon instead of you. Mirror Image is another good option, but few Mirror Image users have good enough weapon attacks to make this feat worthwhile.

Strategies for Maximizing Sentinel Effectiveness

Stand near allies, or stand near enemies who want to reach your allies; this will let you get either those delicious speed-nullifying reaction attacks or those delicious punishment reaction attacks. Keep your DM’s monster strategies in mind; if the way they run monsters lets you use Sentinel a lot, take it, but if the way they run monsters does not enable Sentinel in the slightest, don’t take it.

Final Thoughts on Sentinel

Your mileage may vary on Sentinel, but at most tables, this is a really good feat. I wouldn’t take it before Polearm Master or Great Weapon Master on a strength melee character though; Polearm Master guarantees another attack each round and sometimes gives you a reaction attack for an extra 1-2 attacks per round, and Great Weapon Master sometimes gives you another attack and gives you vastly higher damage in some circumstances.

If you take Sentinel before those two feats, however, you’ll still wind up with a lovely character with unique abilities.