i was a loyal classic windows media player (with k-lite mega codec)user. until i met KMplayer. video quality is basically same but KMplayer has lot more filters that you can mess with. its even better when it comes to audio. tonnes of options you can enable and all my videos sound much much better than ever before. this is the main reason for me to i switch. no installtion needed, unzip it and you can play anything and everything. except flv and rm files though. quicktime i'm not sure.

http://www.kmplayer.com/forums/index.php