Picking the right cantrips in Dungeons & Dragons may not seem important to newer players who think of them simply as their weakest spells. But since cantrips don't consume spell slots or material components, they're one of the most valuable assets a party can have. Cantrip abilities range from saving the day in niche circumstances to serving as a go-to attack in battle.
However, some cantrips are all but worthless by comparison to their alternatives. By seeming deceptively useful, they represent a common pitfall for many newer players. They may be good for flavor, but if you're looking to optimize at all, there are some spells you'd do best to avoid.
Every new player running a spellcaster will inevitably run into the circumstance where they used up all their spell slots and the only magic they can perform are their cantrips. Until that happens, it may be easy to dismiss those cantrips are ornamental flourishes to the character. But once it does happen, players realize just how indispensable those cantrips are. Characters want to be useful in combat and even if somebody thinks of their character as the type to sit out on the sidelines, that will get pretty boring just a few sessions in. But not all cantrips are created equal.
The easiest to compare are the cantrips intended for combat, where objective metrics like damage dice, range, and damage type make some cantrips clear winners and others baffling wastes of space. Eldritch Blast is the gold standard, with a superior damage die of d10, 120 feet of range, force damage few monsters have resistance to, and multiple attacks it makes as it scales up. Compare that to Acid Splash which does 1d6 of damage, has 60 feet of range, and can only hit multiple monsters if they're standing close together, and it's clear that the spell is just flat out worse. And even Acid Splash isn't as bad as Poison Spray.
Poison Spray does 1d12 of damage, which may seem appealing at first glance. The problem is that it requires a constitution save to activate. When it does, the spell comes in the form of poison damage. Constitution is the highest stat for most monsters and poison is one of the most commonly resisted or immunized damage types. Add that all together and Poison Spray will barely ever get the chance to deal that d12. But, what really damns the spell is its range. At 10 feet, a spellcaster nearly necessitates putting themselves in melee to cast the spell, and good luck retreating every turn when almost all monsters double that distance with their speed.
Other cantrips may seem like they'll be useful in battle, but considering alternatives makes their value plummet. True Strike is the perfect example, as it essentially burns an action to give the caster's next attack advantage. But advantage just means rolling twice and taking the highest roll, so rather than casting True Strike why not just attack? Considering that a caster can hit with their first and second attack, True Strike is almost entirely a waste by comparison, and by requiring 30 feet of range it once again puts a caster close to danger.
Spare the Dying is possibly even worse. The spell stabilizes a dying target on touch, which may seem invaluable in a crisis situation where an ally is dying. But when the exact same thing can be achieved with a successful medicine check, or a healer's kit, or virtually any kind of healing, then Spare the Dying pales in comparison. Cantrips need to be common utilities used inexhaustibly whenever possible, not reserved for niche scenarios where several alternatives would be superior.
The common factor in gauging the usefulness of a cantrip here is to consider alternative options. If there are better attacks to make or alternate actions which are superior by every metric, then it's increasingly hard to justify carrying the cantrip. There are those just looking to roleplay as a poison-spraying necromancer or an empathetic healer desperate for all available options. But when it comes to optimization these spells aren't just bad. They're a waste of space.