WMP is crap all around. Too complicated and doesn't look good. iTunes is easy to use but it isn't the best experience on Windows. Sometimes it hangs or lags, others it is fine. My biggest problem with iTunes are the updates, way too big and take far to long (and a restart normally) to complete. Just give me a patch, downloading the entire thing and reinstalling every time just plain blows.
IMO my favorite to actually use right now is actually the Zune software.
iTunes uses a lot of core Apple technology like QuickTime, so it will likely always suffer a little on the PC. I use it pretty much exclusively on both just because that type of interface is becoming so useful and widespread (typical database/datatype metaphor) that it's just a natural. 80/20 rule -- it does most of what I want very easily.
iTunes runs slow on BOTH of my machines regardless of the size of library.
I love the iTunes cleanness and simplicity but I always use WMP when I'm watching movies or listening to music on the computer.
Reason being is that I do not have to allow it to give it permission or add it to the library. I can just double click and it'll play the music. Do note that I'm talking about the basic untweaked usage.
I like WMP more than iTunes. I have to use iTunes because I have an iPhone, but if I would have had another phone, there would be no iTunes on my computer.
