Most difficult that I've finished:
Sardegna 800 when it first opened to me. I ran the Ferrari F-XX-K the first few times. It's always a nail biter with that car. It's easier with more efficient cars.
Suzuka Gr. 1, wow that was tough. I ran a Dallara in that race. The MP4/4 had not been released yet. It took 10-20 tries, rain at the right time, the planets aligning just so, plus some unicorn farts and me hanging on for dear life. I have hated Suzuka since Pole Position 2 and that game is lying there just begging to race.
Most difficult that I have not finished:
The stupid, lame, censored, censored, this is an E rated forum, have you driven a Ford lately, pointless drift missions!
Maggiore endurance race. I get smoked immediately in this.
Best? Honest. Daily Race A. It's an exhibition race and gets you with human players (I see you, Sophy, come on over and race with us!)
Best SP races are anything 700 PP and up that pays 200k or more. That gives you grinds at Spa, Sardegna, Le Mans, Road Atlanta, and St Croix. For some reason PD are just bent on making the Tokyo 600 harder to the point it's no longer any fun.
Also the Clubman race with the Minis is good and the Divine Moves mission is good because the solution has nothing to do with the formula car's capability. You can try passing everyone outright but it's impossible no matter how good you are with that car or in the rain.