I would recommend either
Media Player Classic (MPC), VLC Player or Zoom Player.
These are the BEST media players for video. Have been for years and will be for years.
It is true they have some codecs bundled with them but you should still use a codec pack.
These allow you to play practically every video and audio format out there using direct show. This means that every program you have installed that uses direct show will utilize all the codecs, splitters, filters and everything else in the codec pack.
The best codec pack is the K-lite Codec Pack as djshortbus suggested.
With the install of the k-light Codec Pack it scans the registry and removes other codecs it finds as well as potential conflicts and with a modern day system, a codec pack does not really slow it down.
The best combination is Media Player Classic (MPC) with K-Lite Codec Pack Mega.
I would have put more links for the players but the spam prevention mechanism prevented me from posting more than 1 link. sorry.