I'm stuck in the car for a few hours, and am bored, so I'm gonna write down a list of tracks that I think are good for learning on.
Complete Newbie. So you are completely new to drifting? These are some good spots to start out on.
-Top Gear Test Track: this is a great spot to mess around, practice getting the car sideways. Lots of wide open pavement, and it flat. Don't even worry about following the actual course, smash right through those cones if you want.
-Turn 1, Deep Forest Raceway: this is a great corner to learn how to hold a drift through a complete corner. Start out nice and slow, and just drift the corner both directions over and over again. The rest of the track is actually quite tricky due to the elevation changes and banking, and is more trouble than it's worth for beginners.
-The S Curve, High Speed Ring: this is a great set of corners to learn and practice holding a drift while changing directions. Track is wide, and flat. Again, just drift the S Curve, and forget about the rest of the track.
Beginner. These are some tracks that are good to start out on. Start out by drifting each corner individually. When you feel comfortable with that, start trying to link multiple corners together.
-Tsukuba: the most commonly used drift track in the game. It is a great track to learn on, as it is pretty flat, doesn't have much banking, and has a smooth road surface. It has a nice verity of corners to learn. It also offers opportunities to learn some more advanced techniques as you become more confident. Linking the last corner to the first is as good a place as any to learn high speed transitions and long e-brake, 90', and even reverse entries.
-Autumn Ring Mini Reverse: a pretty low speed track that has lots of good transitions. For starting out, you can practice both directions of the flat, double s-curve section. Then start working your way into the big uphill s-curve. This is a great track to learn how to do complete laps while maintaining a consistant drift. There are a few sections that will require you to learn how to start thinking a few corners ahead, and plan your line accordingly. Personally, I would recommend learning on the reverse config before attempting the forward. I find the banking to be much more forgiving on the reverse.
-Suzuka East Course: a classic drift spot, used by drifters of all skill levels. The consecutive s-curves are great for learning how to maintain a drift, and how to transition smoothly. Remember, you can do the course in either direction. It is actually very common in online lobbies for people to drift only the s-curves, turning around at each end.
-Cirquito de Madrid Full Course: although the entire course is great for drifting, I would suggest beginners avoid the very tight hairpin at the top of the course. The very twisty bit is great to practice quick transitions. It's also a good place to start "street drifting" (ie dfrifting in areas that have walls on both sides as opposed to run-off areas). The carousel at the bottom of the track is also great to practice holding a drift through a long turn.
-Tokyo Bay Cart Track: a lot of new drifters forget about this track, but it is a brilliant place to learn. I would suggest creating your own. Make one that is 3 or 4 sectors, with the complexity of each in the 5-8 range, and make the track as wide as possible. Start with something nice and simple, and add corners as you get better.
-Nurburgring GP, Sector 1: another great place to learn on. The paved runoff areas are nearly endless. If you load the longer GP/F version of the track, the Dunlop Curve (the big 180' right hander) is another great corner to practice on. The rest of the track is only so-so for drifting, and should probably be avoided by beginners.
-Spa (DLC): if you bought this DLC (which you should, it's Spa for crying out load!!), Spa is another great place to learn on. Wide, smooth surface with lots of paved run-off area.
-Fuji Speedway: it's good for pretty much the same reasons as Spa.
-Indianopolis Motor Speedway: the road course, obviously. Again, it's wide, smooth, has no elevation change or banking (in the infield at least), and offers a good variety of corners.
-Eifel Custom Track (not the cart version): the Eifel custom tracks are a great place to start learning how to maintain a drift through elevation changes. Like Tokyo Bay, make the track as wide as possible, start simple, and add corners as you progress.
Intermediate...well, once you reach this level, you should feel pretty confident to start tackling any track in GT5. This is more a list of tracks commonly used in drift lobbies that you'll want to familiarize yourself with. With the GT original tracks, it is common to see people using both directions.
-Autumn Ring Full
-Grand Valley Speedway
-Trial Mountain
-Deep Forest
-Eiger Norwand Short Course (reverse is not too common)
-Special Stage Route 5
-Tokyo R246
The following courses are great drift tracks, but are quite advanced:
-Nurburgring Nordschlief
-Circuite de la Sarthe
-Cote d'Azur
This list is far from official or anything, just my suggestions.