Love:
Sierra: By far my favorite circuit in the game. A combination of sweeping corners, steep hills, and a beautiful lakeside view is a joy to go around, especially in a mid-engined, rear drive supercar (preferably the Ferrari 288 GTO).
Nurburgring Nordelschife (hopefully spelled correctly): No explanation required.
Trial Mountain: I grind the RB Standard around this track, and it is FUN. All those high speed corners and overtaking spots make this my most driven circuit (probably due to grinding).
Ascari: Fast-paced and beautiful.
Monaco: Not so much the circuit as the scenery.
Laguna Seca: Now, I know, what blasphemy, but 90-degree corners can be fun, they just make the course map look ugly. And I love the Corkscrew. And I live in Northern California and I saw a Tudor SportsCar race there so I'm kinda biased...
Sarthe: Flooring a wicked fast LMP through Muslanne is a hoot!
Hate with a passion:
Apricot Hill: I always do horribly at this circuit. The corners seem forgiving, but every time I end up in the gravel.
Willow Springs: Same as Apricot Hill, but replace "gravel" with "sand".