I've been using iTunes with the nifty little Bitperfect app to play my apple lossless files. This may be an old discussion but I recently discovered Audirvana Plus and am finding that it sounds better than iTunes. The music just seems to flow better, better detail, and all around just smoother. They utilize their iZotope 64-bit SRC converter and so far it seems like it's the real deal. What's really cool about Audirvana is that you can integrate it with iTunes. It's $49 but I'm enjoying the trial and may end up purchasing it when my times up.
My system is iMac - Keces DAC - Pass B1 Buffer (pre) - Parasound HCA-750a - Polk Monitor 5 Series II speakers
I'd love to know what audio players you guys are using with the Mac OSX and why.