VLC Media Player
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Full user review
"Easy to use"
Can use almost any codec you use, built in equalizer.
Sometimes going to full screen, it keeps your taskbar on the bottom until you exit full screen and go back to full again.
I've been using VLC for a long time with great results. The built in EQ allows you to boost overall levels on quiet videos (or adjust sound to match your equipment). Also it's very easy to advance or retard the soundtrack in case it doesn't match the video. Easy to use overall.