Having used both Octane for years and Redshift for a while now, it really depends on what you want to do exactly.
As @Cerbera already said, it's very easy to get very good looking results with Octane. In fact, you can add a sphere and an HDRI in Octane, put a material on it and it already looks photorealistic. The settings you can make are very barebones, 90% of the time you can go with default render settings (apart from switching between Direct Lighting and full Path Tracing depending on the scene). However, for me Octane was pretty annoying to work with due to numerous reasons. Yes, it likes to crash although when I still worked with it I managed to reduce the crashes by not using two GPUs. I don't think it was an issue with the two GPUs in general but rather because I used a 2080 Super in combination with a 1060 at the time.
Redshift is increadibly fast and if you're not after 100% photorealism and okay with learning how the render works (which takes some time) I would definitely recommend it. I've had it for a half a year now and I kid you not, it did not crash once even though I tried my hardest to get it to crash. The material system is more complex but way more flexible than Octanes but for basic PBR rendering the two don't differ much from each other.
I have not worked with Arnold so I can't comment on that, but I've only heard good stuff about it. It does seem to be very technical though so if you just want fast results it's probably not the best idea.
So in my mind:
Octane if you want easy photorealism, especially for stuff like product shots
Redshift if you value speed above everything else while not getting the easy photorealism
Arnold seems to be kind of a combination of the two, although I'm pretty sure it will be slower