File format? mkv, so convenient. But media codec would be h265 any day. I find the video quality to file size to be perfect for most films and only have issues with it on the largest files and the lowest power hardware (Roku TV). For the movies I really love and rewatch I sometimes get h264 for the better visual quality. I tried some AV1 files and found the artifacts really ugly, but admittedly these were very small files. That and the lack of hardware decoding on most hardware is preventing me from migrating.