MP4
The container format that works everywhere — not the most capable, but the only one you can send to anyone without thinking.
MPEG-4 Part 14 is based on Apple's QuickTime format and was standardised in 2003. Contents are organised into boxes, where a moov box describes the tracks and an mdat box carries the data itself. If moov sits last, the whole file must load before playback can begin, which is why web optimisation means moving it to the front.
Fragmented MP4 splits the file into independent chunks and is the basis for adaptive streaming in both HLS and DASH. The combination of MP4 with H.264 and AAC is as close as digital video gets to a universal denominator.