I find that Super Speedway is my worst track, because unlike the Test Track, its short and quick, meaning more laps. Even though 6 laps round SS is the same (or nearly) as a couple of laps round the Test Course, Your only seeing the scenery and track of the TC twice while on SS your going round and round...
I love SS11 and Monte Carlo, simply because I find them most challenging, one of my best experiences on SS11 is the beginner Turbo cup in the '80's RX-7, it took hours for me to finally beat the 3000GT, I kep coming 2nd every time and each small supply of money I recieved made that little more close behind the lead car. Anyway, enough of my GT adventures.
As for Monte Carlo, it is one of the most difficult tracks on the game, but that's what I find most exciting about it, I could take my 400R and do tons and tons of laps round it, finding quicker and quicker ways of tackling the course.
Another 2 of my fave course are Deep Forest II and Tokyo, I find Deep Forest to be an amazing track to watch a replay on, especially when your driving in quite a hard race. As for Tokyo, I love the speed of it, It has its hard corners and quick corners, as well as its giant main straight.
Other than Super Speedway, I have alot of difficulty's getting Trial Mountian's I and II, I seem to hit the craggy walls every time on the corners just before the back straight.
-Marshy