Match your ability to an equivalent spell level. Use the actual spell level if it's a real spell.
0Cantrip(Light, Prestidigitation, minor utility)
1-2Low-level(Magic Missile, Detect Magic, Invisibility)
3Combat-defining(Fireball, Lightning Bolt, Fly, Haste)
4-5Encounter-ending(Polymorph, Banishment, Raise Dead)
6-7Adventure-skipping(Disintegrate, Teleport, True Seeing)
8-9Reality-altering(Dominate Monster, Meteor Swarm, Wish)