I see the K-Lite codec pack bandied about anywhere anytime anybody asks for recommendations for a codec pack, or as a solution to playback problems. Sometimes the monstrous “ACE Mega CoDecS Pack” makes an appearance.
When it comes to codec packs less is definitely more, as exemplified by my pack of choice, the Combined Community Codec Pack (CCCP). Originally developed for playing fansubbed anime, it is tweaked and tested for maximum quality and reliability and is able to handle just about anything I care to throw at it. That one of the players included in the pack is the excellent Media Player Classic is just a bonus.
Make sure to read the installation instructions.
The only major formats lacking are RealMedia and Quicktime, and that is easily fixed by installing Real Alternative and Quicktime Alternative (both are designed by KL Software, creator of the K-Lite pack, so they are doing something right). Also, CoreAVC (not free) is an option for slower PCs for decoding H264 content.
Satsuki’s Decoder Pack seem pretty nice, and XP Codec Pack is another viable alternative. They decode more audio formats than the CCCP, so they may be more suitable for some people. Me, I have QMP for all my audio needs, but that’s a topic for another day.
Note: K-Lite has a “CCCP” profile, but it does not work well, and with none of the tweaks, so what is the point? Get the best, accept no substitutes.
Note 2: Graphic shamelessly ripped from the official CCCP wiki.
The latest betas of CCCP contain newer builds of ffdshow with some impressive gains in speed, by up to 20% it seems. Of course CoreAVC is still that little bit faster, but I can now play most hi-definition (1280 x 768) H264 content without slow downs on my 1.5 GHz Pentium M laptop. Excellent!