I don’t recommend downloading codec packs. Sure it’s more convenient than downloading individual codecs on an as-needed basis, but when problems crop up it’s hard to pinpoint their cause.
That being said, if you’re still intent on getting codec packs, we have a good list of them on
this page.
Your question is “which one is the most complete”. That’s hard to say, but you can check their homepages for the list of codecs contained in each. If you’re lazy you can go by download file size, but that’s not a reliable indicator.