Fireball

Fireball is the spell that clears a room, and the moment the table remembers

At the table. Fireball earns its reputation against crowds, not bosses. Take a level 5 wizard with Intelligence 16 (save DC 14) and four Goblin Warriors at 10 Hit Points each. A goblin saves on 12 or higher, 45% of the time. On a failed save 8d6 always kills it; on a success, half of 8d6 still reaches 10 about 96% of the time. Each goblin dies 98% of the time, and all four drop together 93% of the time. Point the same spell at one Ogre and its average 28 takes 41% of 68 Hit Points. That is a decent hit, not a finisher.

The choice that matters is where the 20-foot sphere goes. Count heads before you cast: three or more enemies inside the blast is a good trade for a 3rd-level slot; one is usually a waste. Upcasting adds 1d6 (3.5 average) per slot level, so a 5th-level Fireball averages 35. Spend the higher slot only when those extra points decide who survives.

The common mistake is forgetting the friends. The sphere hits every creature inside it, including the party's Fighter locked in melee, and it sets fire to flammable things nobody is wearing or carrying. Ask the DM where everyone stands before the dice come out.

The human angle. Nobody retells the fourth round of an even fight. People remember the peak: the turn the wizard says "Fireball", eight dice hit the table, and a whole goblin warband falls at once. Kahneman and colleagues found people judge an experience by its most intense moment and its ending more than its length. Fireball is a designed peak. As a DM, let the players have it. Put the crowd in the room when the wizard still has a 3rd-level slot, then describe the smoke.

Source: Kahneman, D., Fredrickson, B. L., Schreiber, C. A., & Redelmeier, D. A. (1993). When more pain is preferred to less: Adding a better end. Psychological Science, 4(6), 401–405. doi:10.1111/j.1467-9280.1993.tb00589.x