Formula GT is my favorite.
Next would be Endurance, and not to knock over cones.
Professional League would probably be next, 10 laps is a good practice run to judge tire wear and plan pit stops for the Enduros, work on setups, etc.
Beginner and Amatuer leagues I haven't raced since I cleared.
I like to Rally occasionally. While they're not true rally cars, the Honda 400R and the RUF CTR 2 are very fun dirt cars as long as you're not looking for competition from the other car.
The only thing I've done in Arcade mode is to open some tracks to run a TK ZZII on for an online series.
The main thing for me is competition. I get bored when there's no one to race against, so just going out seeing how many time I can lap a car is not something I do. The AI isn't great by any means, but I can usually find a car that I have to drive well to win with. If it's not good enough or I make mistakes, it's usually a 2nd or 3rd place finish, which is good enough for me. After I cleared a professional league series, I would rerun it to win the other prize cars, but I'd use one of my F1's to test with for that series. I seriously question all those who say they have no time for a ten lap race. That's less than 15 minutes for most of those, if you don't have that much time, why even bother putting the game in? I can understand someone saying they don't like a ten lap race because they use cars that overpower the competition (not hard to do) because when you do that, it turns into just running ten laps. The Vitz may suck, but that series is very competitive for me. Now don't get me wrong, I love the time trials. Me and my brother in law take turns trying to put up the "ghost" lap. He's a Deep Forest\Trial Mountain kinda guy while I lean more toward a Grand Valley\Seattle style. We both love Laguna Seca.
It was always my recommendation for beginners to run the Miata Endurance race. After they beat it with the LS I'd say now beat it with a different Miata. By that time they can drive a car, they understand lines better, and they realize it's not all about putting up the fastest time every lap, it's about being fast consistently. They also come out of it with a good chunk of change.