WMP started getting decent with v10, now it's awesome, fast, flexible, and as good as anything else really.
I don't really enjoy media center very much, but I don't use a tuner and prefer to just shortcut open the folder I'm gonna play whatever I'm feeling like playing. It's also super easy to manipulate playlists/etc, though I prefer Zune and Winamp as superior music playback software.
iTunes absolutely BLOWS on PC (even overclocked quad i7-860 w/8gb!), but I've seen it running smooth as silk on Macbook Air. I do hate the circa '2001 interface of iTunes though =X It feels super ancient after using Zune v4+, and I bet Apple is gonna massively revamp iTunes soon, it is the one piece of Apple software that I can say is dated, bland dogshit.