Why MP4 + H.264/AAC remains a safe portable-video choice
When broad playback matters more than preserving every disc feature, a conservative MP4 combination remains hard to beat.
The compatibility combination
A common portability-oriented target is MP4 containing H.264/AVC video and AAC audio, usually 8-bit 4:2:0 video and a widely supported H.264 profile/level. This combination is supported across a very broad range of phones, televisions, browsers, consoles and media players.
Why it works
MP4 is a widely implemented container, H.264 hardware decoding is nearly ubiquitous on modern playback devices, and AAC is a common companion audio codec. The combination avoids many compatibility failures caused by less common subtitle formats, lossless surround audio or unusual profiles.
What you give up
A compatibility copy may omit menus, rich Matroska attachments, lossless audio, multiple obscure subtitle formats or specialist metadata. That is acceptable when the source-preserving master or remux is kept separately.
When HEVC makes sense
HEVC can reduce bitrate substantially for similar visual quality, especially at higher resolutions, but older devices are less predictable. A compatibility-first library may therefore retain H.264 while a newer-device library uses HEVC Main 10.
Keep it as a derivative
The strongest workflow is to derive the MP4 from a better master or remux. That way the portable file can be replaced later without re-encoding from an already compressed derivative.