This type of thing is actually really hard to answer definitely. Benchmarks aren't going to show a difference (or at least, not one that maps to your real-world experience) because the raw number-crunching speed of your cpu isn't changing, it's the amount of work performed by the software and libraries in your OS which make a difference.
As a result, you're stuck going off of people's subjective experiences, which is in turn colored by all sorts of things, particularly as most users don't typically go back and fourth between OS versions.
But—since you've blocked out anything earlier than High Sierra, I'm going with High Sierra exactly! We can discard Big Sur and everything after it, because Big Sur came with a clear and obvious speed hit. We can also discard Catalina because it's an awful buggy mess.
That leaves Mojave and High Sierra. Both are solid choices, but on older Intel Macs, I've noticed a slight but perceptible difference in responsiveness between High Sierra and Mojave. My personal belief is this is because Mojave uses Metal more extensively for UI rendering, which should make things faster but which must be less optimized for the GPUs in older models.
