VLC and KMPlayer are the best media players available which uses internal codecs. Both plays almost all videos and audios. But by comparing we'll see some factors such as:
1. VLC is more simple. KMPlayer has lot of features and options but cannot be understood by common people.
2. VLC is more customizable with lot of 'buttons' which makes features(record, snapshot, A-B) easily accessible.
3. KMPlayer supports 3D, but VLC don't.
In comparison i suggests VLC for casual users.
K-Lite Codec Pack and Windows7 codecs are the next suggestions. They are both codec packs for Windows media player in Windows XP and 7 respectively. If you install them you will be able to play almost all kinds of video and audio on your Windows media player. K-Lite codec Pack is for XP, Vista codecs for Vista and Windows7 codecs is for Windows 7.
So KMPlayer/VLC + Windows Media Player with K-Lite Codec Pack/Windows7 codecs/Vista codecs= Every Media and Audio.
Winamp is not needed if you have Windows Media Player.
N.B: Always use latest version of the media player and Codec Packs