VLC
If there’s a video format out there, odds are VLC can play it. From DVD to MKV, VLC can just play it.
VLC also offers features such as transcoding and streaming support for the advanced user. I recently set up VLC as the default player on a computer for my friend’s five-year-old daughter. She can double-click a movie and VLC is set to open full-screen and start playing. No hassle.
If you don’t have VLC, just get it. It’s free, it runs on Macs, Windows, and Linux. It plays almost everything, and it just works. It has plenty of advanced settings if you want them, but mostly, double-click and start watching.