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 did something similar except I wrote a C# program and used AvaloniaUI to build a cross-platform GUI. It was a project to learn C#. I have to make some updates to that now that I think about it...