The point is that with a choice of filters available, directshow players like MC15 and MPC-HC can offer superior playback quality in both audio and video. Because WMC and VLC work as a default doesn't make them better players - if you know what you're doing. Once you're familiar with setting up directshow filters, you'll quickly see what we're referring to. Finding the right combination of splitter, decoder and renderer yields awesome results.
Here's what I recommend currently:
Splitter: Gabest's MPC-HC splitter, now know as Matroska source/splitter.
(Haali will also work well, but you have to be careful about which version and multiple copies being installed simultaneously.)
h264 Video decoder: I'm a big fan of CoreAVC. It's under $20 and does CUDA acceleration if you have a compatible nVidia card.
MPC-HC's standalone video decoder also works well and offers DXVA with compatible nVidia and ATi cards.
FFDSHow also offers a good DXVA comaptible decoder.
Audio decoder: FFDShow audio decoder. Will decode AC3, DTS as well as TrueHD. It'll also passthrough Dolby TrueHD and DTS-HD MA to your receiver as well as the lossy formats AC3 and DTS.
AC3Filter used to be my favourite for just AC3 and DTS passthrough.
Subtitle filter: FFDShow's subtitle filter works very well for me.
Renderer EVR if using Vista or above.
Haali video renderer if using XP
MadVR if you can get it to work.
If you must get a codec pack, I have seen more success with Shark007's than with CCCP. Shark also posts support answers related to his pack on AVSforum. If you have the right hardware and download/buy the above software, you should get a really positive experience. All the software mentioned is free except CoreAVC.