I will use AVC and HEVC terminology for h264/x264 and h265/x265 respectively.

For video, you get jumps of compression quality from AVC to HEVC, from 8-bit to 10-bit, and from HEVC to AV1.

Depending on your source material, changing compression settings, like target quality, variable bitrate, etc, can also have significant gains. It will depend on your source and target though, and may need some testing to get “right”. If you’re looking for the best compression, that may be on a file by file basis, because different kinds of video have significantly different compression behavior or concerns. That’s likely not feasible for a mass of files though.

Playback compatibility should also be considered. AVC mp4 is the most compatible, right now, if you consider all kinds of and older mobile and embedded devices. If you’re fine with modern or desktop, you can go for the best compression codecs.

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To get an idea of encoding time investment and quality, you can use ffmpeg with default quality settings, and target the different encoding targets.

AV1 10-bit, Opus audio:

ffmpeg -i input.mp4 -c:a libopus -c:v libsvtav1 -pix_fmt yuv420p10le out_av1-10bit-opus.mkv

AVC mp4 (when targeting mp4 these codec settings are the default, so in fact don’t have to be specified):

ffmpeg -i input.mp4 -c:a aac -c:v libx264 -pix_fmt yuv420p out_avc-aac.mp4

HEVC 10-bit, opus:

ffmpeg -i input.mp4 -c:a libopus -c:v libx265 -pix_fmt yuv420p10le out_hevc-10bit-opus.mkv