Sadly we are still incredibly lacking in encoding. It's been years since hevc encoding was included in consumer graphics cards and embeddable devices. For one reason or another vendors just haven't done av1 on the hardware level.
I understand that you are probably trying to be funny, but just to be clear, linux iso's are not videos and therefore can't be compressed with a video codec.
Assuming that you actually meant video content, I think your question may be a bit misguided on the nuances and goals of video encoding. Video encoding can be both lossy and lossless. Lossless video encoding isn't particularly interesting in most cases, but I do believe that HEVC (H.265) will usually come out slightly smaller. However anything to do with encoding will always vary based on the actual source content. So partial answer to your question would probably be x265, but it depends. Based on the source you could construct theoretical content that could be better tuned to one or the other's encoding strengths.
Where it gets interesting is in lossy encoding. With lossy encoding you seek to retain visual acuity to a certain standard while minimizing size and/or processing requirements. Both codecs do an excellent job at removing the right amount of information to effectively fool the human observer. With lossy encoding there isn't really a filesize difference, as you tune your filesize to whatever you want to given your source and your desired output constraints. The big feature of av1 is that it is open and unencumbered by patents+royalties and will hopefully therefore make it THE industry standard in the coming years. It's openness also makes it more likely to eventually be ubiquitous as it should be implementable and playable on most new video platforms and hardware, and hopefully the mythical one format that just works everywhere.
For some animated videos, the difference was quite impressive to me.
AV1 100 MiB
H.265 500 MiB
H.264 1.5 GiB
At this point, the audio codecs and track count start making a difference, so this isn't really a fair comparison. And BTW, in terms of video quality in the files above, AV1 > H.265 > H.264.
In non-animated content, the difference is less impressive, but comes at about ~20% in favor of AV1 vs HEVC, in my subjective-and-not-rigorously-benchmarked opinion. But "video quality" is subjective anyway.
Since image quality is subjective it's hard to say exactly, but estimates I've seen is a 20-30% size reduction for the same quality in AV1 compared to HEVC.
Cannot offer any data on that but I experimented with a SD TV series episode. HEVC and AV1 ended up at the same file size (few kilobytes difference) but encoding AV1 took over 10 times as long using rav1e.
Considering lossy encoding, you cant compare file sizes without comparing also quality, or you're just comparing default settings. I can make a tiny mpeg1 file (which will just be a pixel soup)