If you're looking to strictly
decode video, then I'd recommend FFDshow. The latest build can decode almost anything under the sun, including x264, an open source version of h264 (which it can also decode), a very advanced format which the anime community has been adopting, lately. The only problem with FFDshow is that it's not really a codec, but a DirectShow filter, so you may have problems decoding videos with anything other than a video player (such as an NLE). The
2005-20-18 version from free-codecs.com is not a bad start...
You might still need a stream splitter (but usually not) and
Haali's is as good as any, maybe even better.
Still, if you wan to decode quicktime and real media, I'd recommend
QuickAlternative and
RealAlternative. They will let you play QuickTime and Real Media movies, respectively, with any Windows media player...
Good luck with whatever you choose!