Much Ado About Nothing
After reading so much of Forums, Reference materials like Wikipedia, About.com and after spending a lot of time hunting for a Best Codec Pack the Question of "Best codec pack for windows Os (XP, Vista, Seven)" seeems rather odd. But one can ask the question of which is the Best Codec for Coding/Decoding a Multimedia File.
Codec Packs: After all they are just Codec packs, ie pack containing all the necessary Codecs from different sources to help the amatuers, average user & lazy proffessionals to play the multimedia rather than hunting for a specific codec for playing a file
Wikipedia: Codec Pack is a collection of audio and video components for Microsoft Windows that enables the operating system and its software to play back various audio and video formats generally not supported by the operating system itself (little edited)
About.Com: Codec packs are collections of codecs gathered in single large files. There is much debate over whether it is necessary to get a large group of codec files, but it certainly is the easiest and least-frustrating option for new downloaders
So no big deal here and my suggetion is just instal the one which meets the following Basic Critteria
1) Plays most of the formats if not all the Formats available currently,
2) Free of malware/adware/spyware and simillar crap,
3) Reasaonably stable and makes least Changes to OS
4) Easy to Install or Unistall alteast for Amatures & Less Technical average users
5) For Professionals and Advanced users - Individual Preferences like Advanced options etc.,
I guess most of the popular packs meet the above criteria since each one tries to excel the other, but nothing is perfect each one has its own Merits and Demerits. From my personal expereince i have tried a lot of codec packs finally i settled with K-Lite Mega Codec Pack since it met most of the above criteria. I have not tried Shark's Codec packs, will do so in the Future.
Here are the codec packs recommend by About.com: (
What Exactly Is 'Codec', and Why Would I Ever Need It?)
1. CCCP Combined Community Codec Pack
2. XP Codec Pack
3. K-Lite Codec
4. K-Lite Mega Codec
As for Mr.Shark who's is to be appreciated for pains takingly creating a codec pack and spoon feeding it to most of the users - hats off to you

, But his emotional outburts come a like stone to the face of those who make casual remarks about his works making him Mr.Stony Shark 007 - sorry Mr.Shark just kidding

. If Mr.Shark were to remain cool

and diplomatic

to his critics it would only add to his reputation. A Person like you will definitly recieve negative remarks just go easy on them, show your skill in your works instead of insulting them that would shut their mouth

, wouldn't it Mr.Shark ? And Mr.Shark you keep on metioning K-Crap if it is a crap to you so is your's to others, learn to respect others crap as you would want them to respect yours. I'm just trying to be fair here thats it

.
As for a the Best Codec is concerend its goes by Bit Rate, Lossy codecs: / Lossless codecs, In case of Lossy codec the comppression ratio, whether it is an open source or not and so on.
See Wikipedia:
Comparison of Codecs
http://en.wikipedia.org/wiki/Comparison_of_video_codecs
http://en.wikipedia.org/wiki/Comparison_of_audio_codecs
Also See
Video compression codecs, how to compare video codecs the right way
This is the head ache of the person who is Creating and or Distributing the multimedia file to select a particular codec for compressing his File. The end user need not be worried about it because you just download the file that is already encoded with a certain type of codec, if you are not saftisfied by the quality of the video go for a different one or you don't have much choice here.
Article From About.Com -
What Exactly Is 'Codec', and Why Would I Ever Need It?
What Exactly Is 'Codec'? Why Would I Ever Need It?
About.com Internet for Beginners
By Paul Gil, About.com Guide
Question: What Exactly Is 'Codec'? Why Would I Ever Need It?
Answer: (February, 2010)
"Codec" is a technical name for "compression/decompression". It also stands for "compressor/decompressor" and "code/decode". All of these variations mean the same thing: a codec is a computer program that both shrinks large movie files, and makes them playable on your computer. Codec programs are required for your media player to play your downloaded music and movies.
"Why do we need codecs?"
ANS: Because video and music files are large, they become difficult to transfer across the Internet quickly. To help speed up downloads, mathematical "codecs" were built to encode ("shrink") a signal for transmission and then decode it for viewing or editing. Without codecs, downloads would take three to five times longer than they do now.
"Is there only one codec I need?"
ANS: Sadly, there are hundreds of codecs being used on the Internet, and you will need combinations that specifically play your files. There are codecs for audio and video compression, for streaming media over the Internet, videoconferencing, playing mp3's, speech, or screen capture. To make matters more confusing, some people who share their files on the Net choose to use very obscure codecs to shrink their files. This makes it very frustrating for users who download these files, but do not know which codecs to get to play these files. If you are a regular downloader, you will probably need ten to twelve codecs to play your music and movies.
"How do I know which codec to download and install?"
ANS: Your Windows Media Player will often try to communicate to you the 4-character code of the specific codec it needs. Note this code, then visit this website
http://www.fourcc.org/fcccodec.htm1 to obtain the missing codec. For a small FAQ section, follow the link on the left called "Sample Code".
"What are the codecs I should download and install?"
ANS: There is no single best answer to this question. There are so many codec choices. The easiest option is to download "codec packs". Codec packs are collections of codecs gathered in single large files. There is much debate over whether it is necessary to get a large group of codec files, but it certainly is the easiest and least-frustrating option for new downloaders. Here are the codec packs we recommend at About.com:
1. CCCP2 Combined Community Codec Pack is one of the most comprehensive codec packages you can download. CCCP was put together by users who like to share and watch movies online, and the codecs they've chosen are designed for 99% of the video formats you will experience as a P2P downloader. It is still virus-free as of February, 2010, so definitely consider CCCP if you think your computer needs updated codecs.
2. XP Codec Pack3 XP Codec Pack is a sleek, all-in-one, spyware / adware free codec collection that also offers a good, solid Media Player Classic. Currently just under 6MB in size, XP Codec Pack is truly one of the most complete assemblies of codecs needed to play all major audio and video formats.
3. K-Lite Codec Pack4 Very user-friendly and well tested, K-Lite Codec Pack is loaded with goodies. It will enable you to play all the popular movie formats. K-Lite comes in 4 flavors: Basic, Standard, Full and Mega. If all you need is to be able to play DivX and XviD formats, Basic will do just fine. Standard pack is probably the most popular - it has everything an average user needs to play the most common file formats. Full pack, designed for power users, has even more codecs plus encoding support.
4. K-Lite Mega Codec Pack5 Mega is a very comprehensive bundle...it has everything but a kitchen sink. Mega even contains QuickTime Alternative and Real Alternative.