The hardest event for me was back in GT2, a mid-engined race that took place at Rome. The fastest opponent was always an old GT40 and for the life of me (I was about 12 at the time) I couldn't find a car that would beat the GT40. Of course, I had no idea how to tune suspensions or transmissions back then so the event probably wasn't all that hard if you did it right. My young self just couldn't beat it with basic power and bolt-on suspensions. To this day I've never beaten it but I haven't tried since then either.