Aqualung for me, but I don't use 90% of its features. It was the first thing I found that had a good-enough GUI but didn't try to force music library functionality on me (it has it, but it's willing to let me leave it off and just use the directory structure I long ago optimized for the weird nature of my collection).

I use Guayadeque for basically one reason only: it supports multiple libraries/collections. I wish more players had this feature. My collection is basically unmanageable without it, and it's large but not that large (about 30k tracks). Are they any other players that support multiple libraries?