Mine is Strawberry since it has a ton of options and plays a ton of formats. It's also (distant) fork of Amarok 1.4 and integrates well with KDE Plasma. I'm curious what other people are using these days. What's your favorite player?
I was using Clementine for a long time and switched to Strawberry about a year ago.
Since they're related, migrating libraries from one to the other was also possible.