Range fit
The song should not force extremes.
The best practice song is not always the hardest song. It should help you work on pitch, breath, tone or confidence without overwhelming your voice.
If you want pitch control, choose a clear melody. If you want breath control, choose phrases that are long but manageable.
The song should not force extremes.
Pick one main thing to improve.
Use lines that challenge but do not overwhelm.
The chorus should be useful to repeat.