There's no real rule for what makes a good reference track. It’s up to you and your taste, but there are a few things to keep in mind:
- Don't pick a track because you like the lyrics or because you love the synth melody. These qualities won't translate to your final master.
- Use a track that has an overall frequency balance, stereo image, and level of dynamics that you want for your own master.
- Use high quality file formats like WAV whenever possible. Avoid using lower bit-rate and lossy formats like MP3.
Learn how to prepare your tracks for mastering.