I'm only level 22 so I haven't raced on every track more than a few times (certainly not on every variation). That being said, I already am annoyed with Cape Ring full. It doesn't feel real at all, it's frustrating, and the laps take an enormous amount of time. It's okay for a circuit to be long, but it must be enjoyable enough to keep you driving hard on each lap. After doing one lap of Cape Ring Full, I just keep looking at the lap indicator then at the time indicator and back at the lap indicator. It's just frustrating to a fault.
Some of the tracks require a certain type of car to enjoy. Tsukuba for instance is very enjoyable on small-engined FF cars. Indy is actually quite challenging when everyone has the same car (like in NASCAR or Indy 500); unfortunately in this game that is rarely the case. A track like de la Sarthe is fun in really high performance race cars, but in anything else, the straights can be dull.
My least favorite tracks are definitely the rally tracks. I personally love rally racing, and I've played DiRT and DiRT 2 for years now. The series isn't incredibly realistic, but it's incredible fun and a challenge. I'm not sure what the rally tracks in GT5 are supposed to represent, but they're neither realistic (especially for WRC) or fun. I think it's clear the GT series should stick to tarmac.
My favorite tracks are Nordschleife (incredible history, but it's also incredibly difficult so you feel accomplished just making it to the end), Laguna Seca (it feels like you're always leaving a turn into another one, which means you have to make difficult passes to win), and Monza (solid technical track of appropriate length with high speed straights and slow, medium, and high speed turns that give your entire car a workout).
My testing course of choice is Monza. After just one lap I feel like I've learned everything the car can do and what I need to work on.