Mission 34. No question. I remember most people couldn't beat it. And if I remember correctly the PAL version was easier to accomplish the mission. Can't remember if it was the time that was lower or the car was faster. I found that out when I went online looking for tips on how to beat that.
Another License test that I found hard was the GT-one around Monaco..that was when I started using manual shift - I learned with that track/car combo, and I found out it cut my time down significantly.
There was one more license test I vaguely remember - it was GT1 or 2 and you came down a big hill, went up a hill and at the top turned left...don't even know the track or car, but it was bloody hard to get gold. It can still visualize that track