I use VLC media player but I'm not sure if I would call it the best, because I haven't tried them all. I use it because it plays every format that I've tried with it and it's free.
No, this is for Windows. Actually, it works for about every OS, except MAC. At least, I'm not aware of it being ported to MAC. You can read more about it here. http://www.videolan.org/vlc/
if u want good vid quality then the media player wont help but if u want large codec support and filter options (which might be what ur referring to when u say quality) then just get CCCP