I use 100 for aggressiveness and I typically run a field of 30 cars, so 29 AI at 97% difficulty
I like door to door racing and being put under pressure from behind so this suits me. Given the slightly higher incidence of love taps my SR is disabled for offline racing.
90%-95% works for me - I vary depending on the track.
Over 95% you will lose Safety Rating - if you don't care about it that is fine. I didn't even know you could disable it for offline racing .
Lower than that makes the AI too predictable and stagnant imo.
way back the developers told us to set it at 95% or above to give the competition more of an edge. a bit later the "disable SR offline" was introduced ... I have set mine at 98% and have the SR enabled and cannot complain about their behaviour. Below 90% i found them very dull but that was quite some time ago, maybe developers have changed the behaviour in the meantime.
If i remember correctly, i read in an interview with Stefano about the AI, that the aggressiveness ist just the value of how likely the AI is to leave the racing line.