HandBrake

HandBrake is the tool you reach for when a video won't play on your device. It converts between virtually every format and codec, with built-in presets for Apple devices, Android, web, and more. Despite its power, the UI is approachable: pick a file, choose a preset, and hit start. Advanced users can tweak every encoding parameter. Free, open-source, and actively maintained for over 15 years.