SOURCE: Player’s Handbook

Resilient’s Two Benefits

Benefit #1 – 

+1 to an ability score

A common and useful bonus in many feats, best used to turn an odd ability score into an even one. If your goal is to boost your saving throws, then using this to bump up an odd score will be handy. This can also improve things like skills, hit points, initiative, ordinary ability checks, carry capacity, spell save DC, and all of the other things that increasing an ability score modifier usually grants.

Benefit #2 – 

Proficiency in that ability score’s saving throw

Proficiency in a saving throw is always useful. Remember that the +1 and the proficiency must both be for the same ability score; you can’t pick a +1 for intelligence and proficiency in constitution saves, for example.

Mechanics and Requirements

The Boost to Your Saves

Proficiency in an entire ability score of saving throw may or may not be great; if you take this feat at level 4 and increase an even ability score (so no modifier increase), you get +2. Take it at level 19 on an odd ability score and you get an astronomical +7: +6 from proficiency bonus, and +1 from turning an odd score into an even one.

I would not spend a feat to get a +2 to a saving throw unless there was absolutely no other feat or ASI I wanted. A +4 bonus is starting to look appealing however, and that almighty +7 is indeed extraordinarily tempting; it beats the bonus from a paladin’s aura of protection, even if it’s only for a single saving throw type.

The Three Stats

You should almost never use this feat to gain proficiency in Strength, Intelligence, or Charisma saving throws. Dexterity, Wisdom, and Constitution are the most common saving throws by far, and the bonus benefits from increasing an odd ability score (initiative, perception, hit points, etc) are useful as well.

You can still justify going for the three more uncommon saving throw types if your DM uses specific types of monsters; a psychic mind flayer themed campaign would absolutely make taking Resilient in Intelligence worthwhile.

However, keep in mind, you can only take Resilient once! The general non-repeatability of feats still applies, and Jeremy Crawford has confirmed that you’re not supposed to take it more than once.

Of the three common saving throw types, constitution is likely the best proficiency if you cast spells that require concentration, followed by wisdom, on account of how crippling wisdom save effects tend to be. If you fail a dexterity saving throw, you take some damage. If you fail a wisdom save, you sometimes lose your entire next turn, possibly multiple turns, or you could even be turned into an enemy of the party via a dominating effect.

Taking Resilient in dexterity is still appealing, but keep in mind spells like Absorb Elements already protect you from a large portion of dexterity saving throw effects

Ideal Characters for the Resilient Feat

Top Classes

Cleric – Resilient (Constitution) on a level 5 or higher cleric will allow them to run into melee and maintain concentration on the Spirit Guardians spell fairly easily. Spirit Guardians is a great source of damage, and its only weakness is that you might lose the spell by failing a concentration check and running into melee tends to provoke a lot of those. You nullify your biggest weakness by doing this; Resilient (Constitution) could even be worth taking at level 4 before increasing your wisdom.

Barbarian – Barbarians are good at hitting stuff, so a particularly evil and powerful enemy may seek to turn them on your adventuring party via a Dominate Person spell, or they may settle for just paralyzing the barbarian to get them out of the way. At high levels, it’s not uncommon to run up against DC 18+ wisdom saving throws: you need proficiency and some other bonus like a paladin’s aura to have a good chance against them. However, this is not necessary if you went with the Path of the Berserker, which has a special feature that can rival or even exceed Resilient (Wisdom)’s potential.

Artificer – Several subclasses of artificer have massive armor class potential built-in; the Battle Smith comes with the Shield spell and heavy armor proficiency, and the Artillerist gets the spell too. Combine with medium/heavy armor and a shield for 24 or 25 armor class easily. However, dexterity saving throws will ruin your day, and many DMs will increase the frequency of dexterity saving throws when faced with a high AC character; take Resilient (Dexterity) to negate incoming Fireballs and dragon breath weapons.

Taking Advantage of Multiclassing

All classes require a 13 in one or two ability scores to multiclass into them; why not turn that 13 in a constitution/wisdom/dexterity class into a 14? Particularly appealing for Resilient (Dexterity) users with medium armor proficiency since 14 is the optimal dexterity for using medium armor. 

Useful Race Advice

If you want to protect your wisdom saving throws as a barbarian, artificer, or any other class without proficiency in it, and you don’t want to multiclass, then a race that gives advantage on the relevant saving throw is fantastic. 

Kalashtar – has advantage on all wisdom saving throws; combine with proficiency for amazing saving throws. 

Satyr – has advantage on all saving throws against magic, and most dangerous wisdom saving throws involve magic. 

All elves – have Fey Ancestry, a feature that gives you advantage against being charmed, so if you specifically want to avoid charm effects, going with any elf subrace should do.

Combos, Tactics, and Synergies

Complementary Feats

War Caster will turn your concentration into an unbreakable rock if you take it with Resilient (Constitution). Lucky does a similar thing with all saving throws; remember, a reroll is more useful when the chance of that reroll succeeding is high.

Spells that Synergize? Anything?

No spells really synergize with Resilient.

Strategies for Maximizing Resilient Effectiveness

This mostly applies at high levels, but stand near your party’s paladin! Having proficiency, a high ability score, advantage from some source, and a +5 from the paladin will turn the ordinarily terrifying DC 20+ saving throws from D&D’s deadliest monsters into jokes.

Final Thoughts on Resilient

It’s a good feat, and best at higher levels, even though it does nothing when the relevant ability score isn’t being tested by a saving throw. However, despite being boring, it can prevent worse boredom; being paralyzed for five rounds is one of the most boring things in D&D, after all.