AV1 / Opus where we can. But H264 is far better supported. For example, Safari doesn't include a software AV1 decoder. So AV1 videos only work in safari on M3 or later laptops, and iPhone 15 or later phones.

H264 is the compatibility king.

https://caniuse.com/av1

Safari, or better, macOS and iOS include a software AV1 decoder (libdav1d), but it's used only to decode avif, and to generate file previews in Finder.