Magic Missile

Magic Missile never misses, and that is why players love it

At the table. Three darts, each 1d4 + 1 Force damage, no attack roll and no saving throw. The average is 10.5, and the whole range sits between 6 and 15. On average damage it ranks 6th of 15 1st-level damage spells in the SRD, but every point of it is guaranteed.

The choice that matters is how you split the darts. Each dart does 2 to 5, so one dart always finishes anything on 2 Hit Points or fewer. Three wounded goblins on 2 HP each? Three darts, three kills, no dice to fail. Put all three into one creature only when you need that exact total: three darts reach 10 or more 69% of the time. Upcasting adds one dart per slot level, so a 3rd-level Magic Missile throws five.

The common mistake is firing it at a caster who knows Shield. Shield's reaction can trigger on being targeted by Magic Missile, and the target then takes none of its damage. Against enemy wizards, save the slot or bait the reaction first.

The human angle. A 95% chance feels much worse than a sure thing, even when the maths is close. Kahneman and Tversky called this the certainty effect: people give outcomes that are certain more weight than outcomes that are merely probable. That is why a player with a 65% Fire Bolt still reaches for Magic Missile when the dragon is on 4 Hit Points. It buys the one thing dice never sell: knowing. DMs, when a player spends a slot for certainty, narrate the certainty. Three darts, three flashes, down it goes.

Source: Kahneman, D., & Tversky, A. (1979). Prospect theory: An analysis of decision under risk. Econometrica, 47(2), 263–291. doi:10.2307/1914185