Picking the segment matters more than the song choice
A ringtone typically needs to be short — often 15-30 seconds — which means you're not using the whole song but a small excerpt, and that excerpt needs to work as a self-contained loop or a recognizable hook on its own, not as part of a larger musical arc. A quiet intro or a slow verse might work beautifully in context of the full song but sound thin or unrecognizable as a 20-second ringtone clip; the chorus or hook — the part most people would hum if you asked them to sing the song — is almost always the better choice.
Loop points need the same care as any other cut
If the ringtone is set to loop (playing repeatedly until answered), the seam where it repeats needs the same zero-crossing and natural-gap consideration as any other audio cut — a poorly chosen loop point creates an audible click or a jarring restart every single cycle, which is far more noticeable and annoying in a ringtone that might play a dozen times before being answered than it would be in a one-time playback.
Fades make a short clip feel intentional rather than chopped
Because a ringtone excerpt is, by definition, pulled out of context from a longer piece, a very short fade-in and fade-out at the start and end helps it feel like a deliberately crafted clip rather than an obviously chopped-out fragment — this is the same fade principle used for cuts and edits generally, just applied to a piece of audio that's inherently more likely to have an abrupt boundary given how it's created.
Format and length limits vary by device
Different phone platforms have different requirements for ringtone format (commonly M4A/AAC for one major platform, MP3 or OGG more broadly supported elsewhere) and maximum length, and exceeding these limits is the most common reason a custom ringtone fails to set correctly or gets rejected during import — checking your specific device's requirements before finalizing the clip avoids having to redo the edit after a failed import.