Depends what car I'm trying to test, but usually Tsukuba. I know it like the back of my hand (who doesn't?), so I can tell when it's me or the car that's interfering with a good lap time. The Nürburgring is too long for me, one mistake in a lap and I have to spend another 8 minutes to do another, and I make at least 1,000,000 mistakes in a single lap.
If I'm testing a Group C or something really, really fast, Tsukuba isn't much good, though. Suzuka is a good test course, it has a fairly long straight, some high and some low speed corners, and plenty of undulation.