Compatibility is more than an extension
A file extension identifies the container, while codecs describe how the picture and sound are encoded. MP4 with H.264 video and AAC audio remains a broadly supported combination.
Modern formats can be smaller
Newer codecs can deliver similar quality with less data, but older televisions, car systems and editing applications may not support them.
Audio-only playback
MP3 is the safest general choice for older players and car stereos. M4A with AAC often provides efficient audio on current phones and computers.
When playback fails
Try a current browser or media player first. If the device is older, convert a copy to MP4 with H.264 and AAC rather than changing the filename extension.