MP4 is the dominant video container on the web, H.264 video plus AAC audio, in a structure designed for streaming. Every browser, mobile device, smart TV, and editing tool reads MP4. The format is technically a container (not a codec), so two MP4 files can have very different internal codecs, but the H.264+AAC default is what enables universal playback.
How to open
Every video player on the planet, VLC, QuickTime, Windows Media Player, browsers (HTML5 video), iOS, Android, smart TVs, game consoles. Universal.