Well first, it's a codec pack, not a program. Although most codec packs do include media players..
I used to use K-Lite, but I don't feel its the best anymore.. plus it has too many options, you can end up installing conflicting crap sometimes or other problems if you don't know what you're doing.
I recommend
CCCP I use it and have never come across something I couldn't play, except for stupid stuff like Quicktime/Realmedia, but I don't use them anyway.
Codec packs come with all kinds of audio and video codecs (some you can encode with, some other tools are necesarry) and CCCP is pretty easy and straightforward to setup. It's probably more popular than K-Lite these days and is actively maintained