iTunes renders everything through QuickTime - so what QuickTime supports, iTunes will.  If you want, try out MediaMonkey - it has an interface similar to iTunes, and supports most of Winamp's plugins.

As for MPC being the best music player, Winamp can also render through DirectShow, so Winamp can actually support more formats than MPC (you may have to fiddle with the DirectShow plugin though).

Mainly, iTunes doesn't support OGG Vorbis (which I rip all my CDs to) and HE-AAC :+-: PS