I use mp3. I have a large library, and I'm not going to transcode it to something else.
The things I buy are usually mp3 too.
Currently on Snow Leopard I use Clementine. I'd like something else. I'd prefer something that was a bit more stable with a large library; that was really easy to "vote" a track up or down (I have a lot of tracks that I won't listen too. I need a way to mark and delete them. Really delete, not just remove from a playlist.)
I'm thinking of someway to improve the naming and tagging of the files.
Really, it's a mess.
Portable use I have an old Sansa Fuze which I really like. Except the scroll wheel has a well known "getting stuck" problem.