you really have only one choice
ffmpeg libavcodec
no matter its perian, vlc, mplayer, they all use same codec libaray.
As well as Perian Flip4Mac is also an essential so you can play wmvs etc.
since forever?Since when?
Last i knew, VLC brought its own codecs along with itself. I never really used Mplayer, but Perian gets 'installed' to the Quicktime Components folder, which even after installing VLC, does not have stuff in it.
since forever?
i guess you are confused about codecs and codec library.
http://wiki.videolan.org/VLC_Features_Formats
and http://perian.org/#develop
most vlc, perian's codec are support by ffmpeg library.