Bless

Bless is worth a flat +2.5 to three people, on every roll that matters

At the table. Bless takes your action and your Concentration for up to a minute. Up to three creatures you choose within 30 feet add 1d4 to every attack roll and saving throw until it ends. The d4 averages 2.5, so each blessed roll is 12.5 percentage points more likely to succeed, as long as the roll needed isn't already a 2 or a 20.

What changed from 2014. Very little. In SRD 5.1 the target "can roll a d4"; in SRD 5.2.1 it simply adds 1d4. The material changed from holy water to a Holy Symbol worth 5+ GP.

How much it buys. We counted every d20 face against every d4 face for a fighter with +5 to hit:

A +2 save against DC 15 goes from 40% to 52.5%. With three allies making one attack each against AC 15, Bless adds about 0.375 hits a round, and that's before a single save it turns.

The choice that matters is who gets it. Pick the three people rolling the most dice: the fighter with two attacks, the rogue who needs to land one big hit, and whoever is most likely to face the dragon's breath or the mind-bending save. If you're a paladin, you can bless yourself.

The common mistake is casting it once the fight is already going well. Its value is in the first round, before the saves and the hits start deciding the fight. The other mistake is forgetting it's Concentration: one failed Constitution save after a hit and the whole party loses it.

The human angle. An extra die in your hand feels like more control than a flat +2 would, even though the maths is close. Ellen Langer showed people treat chance as controllable when the task has familiar skill cues, like choosing or handling the object themselves. Bless gives every player one more die to throw, and players like that more than they can quite justify.

Source: Langer, E. J. (1975). The illusion of control. Journal of Personality and Social Psychology, 32(2), 311–328. doi:10.1037/0022-3514.32.2.311