Depends on whether you want lossless or lossy. For lossy, OOG Vorbis is the way to go if you want to support open source. If you don’t care about that, WMA is a proprietary format alternative. Both WMA and Vorbis have better quality than MP3, with equivalent file sizes. I use Vorbis myself, because it’s free.
If you want lossless, idk, because I don’t fuck with that.