H.264
The video codec that became universal — twenty years old, worse compression than anything newer, and still what you pick when something must work everywhere.
Standardised in 2003 and also known as AVC or MPEG-4 Part 10. The breakthrough came when Blu-ray, YouTube and the iPhone all chose it, giving it a reach no successor has matched. Hardware decoding is in practically every device made since 2010.
The licensing model is administered by the MPEG LA patent pool, with fees for encoders and decoders but not for streaming content — a trade-off that made the spread possible. That licensing question is the entire reason VP9 and later AV1 exist.