Shield

Shield turns a hit into a miss after you see the roll

At the table. Shield is a Reaction, cast when an attack roll hits you. You get +5 AC until your next turn, including against that attack, and Magic Missile does nothing to you. Because you cast it after the hit, you know whether it will work: if the attack total beats your AC by 4 or less, Shield turns it into a miss. A natural 20 still hits.

How often does it work? We counted every d20 face:

Shield always saves exactly five d20 faces, so it cancels a bigger share of hits the higher your AC already is. A wizard with Mage Armor gets more from it than one in robes.

The choice that matters is which hit to spend it on. The +5 lasts until the start of your next turn, so it also covers every later attack that round. Cast it on the first hit from a creature with several attacks, and the rest face AC +5 too.

The common mistake is casting it at a hit that beats your AC by 5 or more. Ask the DM for the total first.

The human angle. Most of a caster's time is spent watching other people's turns. Shield is agency during the monster's turn: a real choice, with the odds in view, that changes what happens. Ryan, Rigby and Przybylski found that feeling competent and in control of your choices predicts how much people enjoy a game and want to keep playing. Shield delivers both in a single word.

Source: Ryan, R. M., Rigby, C. S., & Przybylski, A. (2006). The motivational pull of video games: A self-determination theory approach. Motivation and Emotion, 30, 344–360. doi:10.1007/s11031-006-9051-8