I think there are a few spells in the PHB that are absolutely perfectly designed, in my opinion. I'm curious if there are others some would like to share and elaborate why they're so great. The spells don't have to be the most powerful, in fact, broken spells are often thought of as poorly designed.
One spell I think is one of the best is a simple cantrip: Vicious Mockery.
It is one of the few psychic damage effects allowed to players and it immediately sets apart a bard from any other caster. It isn't so powerful that a bard should spam it even at level 1, but it's effects are good enough that even a high level bard might consider casting it in high-level combat.
Finally, it leaves a huge "ad-lib" fill-in-the-blank that never disappoints on the roleplay aspect. If your player says a truly insulting injury, it feels like the actual player did the damage rather than their character. If your player says a bad joke, the creature can be measurably damaged from the pure second-hand embarrassment. And it's always entertaining to see a creature die just by saying something so hurtful that it collapses in shock.
Even if bard isn't your favorite class, you have to admit that it's unique spell is very well-designed and fun to use.