For videos - KMPlayer. Build-in codecs (plays mov, rm, nsv, flv...) and a very good flexability in terms of control, you can tune it up very precisely that suits better your taste.

For the audio I cannot decide. Maybe iTunes. I like its interface and ergonomics but it's really bloated in the best traditions of WMP. So to escape iTunes "heaviness" I oftenly use AIMP.

Also foobar is good in terms of sound quality.