VideoLAN or VLC is a compact lightweight video player that Windows, MacOS, and Linux/Unix.
I’ve not found much that VLC won’t play. It’s by far the most stable video player I have found. It also does some other very nice and unexpected things: you can get it to stream video to somewhere else over the net. It’s not meant to be used as an appliance, it’s a little hard to get this set up, but in a pinch, if you need to stream something from the other room for some reason, it works!