A mixing reference is a music piece that you know its details and characteristics very well and can compare with your own mix to help you spot the flaws of the track. Therefore, in this case, you should always choose a reference track that you are familiar with and has similar aspects to the music you are working on.
A reference track is also a good tool for you to get familiar with your sound systems. For example, the speakers that you recently bought. You can play your reference tracks on them and find out their sound specificities like frequency, image, and so on.
Just for myself, I didn’t intend to build my own library of all the reference tracks. I pick different songs specifically every time that is just what I want my own project to sound like. For instance, when I was making Winter Afternoon which is a Progressive House song that features the dark-themed drop with mainly bass playing as the lead instrument, I used Eric Prydz’s remix for CamelPhat’s Breathe and Solee’s Watoto as reference tracks. It really helped me with balancing the different instruments in the song and knowing how to control the compression in the track to make it groove.