A lot of people recommend the Media Center player in MCE 2005, using the Nvidia Purevideo dvd decoder. You can also use that decoder in Windows Media Player. I've tried it and the latest version of PowerDVD, but I can't tell much difference personally. And I have an Nvidia video card that is fully compatible with the Nvidia decoder's features. I've gone back to PowerDVD for the most part.
I've read conflicting reports on using Nvidia PureVideo in Vista so far.