On a bike
Laguna Seca: Awesome, iconic track. Going through the corkscrew is a thrill and more than a little intimidating. The main straight isn't all that straight and crests a hill blindly. Very exciting. Most of the turns are quite fast, very fun track.
Sears Point (nee: Sonoma Raceway, nee: Infineon): Very technical, off camber turns, lots of elevation changes, esses, and of course the bus-stop chicane. Its a great place for a CSS event because it has everything, its near impossible to get through turn 2 without perfecting the basic visual skills, two-step and three-step. I did level 3 there and I'm glad I did level 3 on that track.
Miller Motorsports Park - East: Very fast and flowing track. Astounds me that there are 13 (or 14 depending on how you count) turns on just the east side, I can't imagine how challenging the full track must be. Visual skills here can be quite challenging because the environment around the track is desert and can be very similar from turn to turn. The track is big and wide so its a nice place to learn as its pretty easy to stay out of each others way.
Streets of Willow: Very tight, bumpy and technical. Some elevation changes and surprising number of fast corners for such a tight track. I'd have to defer to the instructional staff but I suspect that if you are quick here you are going to be quick anywhere.
In a Car
Thunderhill: It's been a while, but I enjoyed Thunderhill. Its a small, humble facility but very fun.
Pahrump: Reminds me of Thunderhill, humble but very fun, though probably not worth a trip just for the track. Woth it if you are already in Vegas though.
Portland International: Meh, as much as I'd like to sing its praises (because I love that city) this isn't much of a track.