How to set a ringtone for one contact

A different sound for your partner, your mom or your boss means you know who's calling before you look at the phone.

Want a song or a funny sound for them? Make the ringtone first.

Open Ringtone Maker

iPhone

Add your new sound to the iPhone first (see set any song as your iPhone ringtone). It doesn't have to become your main ringtone.
Open Contacts (or Phone → Contacts) and tap the person.
Tap Edit → Ringtone, choose the sound and tap Done.
Want their texts to sound different too? Set Text Tone on the same screen.

Samsung Galaxy

Open Contacts and tap the person.
Tap Edit → View more → Ringtone.
Tap +, pick the file from Downloads and save.

Google Pixel and most other Android phones

Open the Contacts app and tap the person.
Tap the ⋮ menu in the top right → Set ringtone.
Choose My sounds or scroll to the bottom and tap Add ringtone, then pick your file.

Ideas that work well

A short clip of their favorite song, their laugh from a video (see make a ringtone from a video), or a slowed + reverb version of "your" song. Pick something that's recognizable in the first two seconds, since that's often all you hear.