I use SSR5 extensively (and barring some Le Sarthe II gear ratio testing, exclusively) for all of my testing. Barring road bumps (which, being a highway, it lacks), I feel it models elevation changes, multiple corner sharpness-es and speed sections the best of any track, and it is short enough to not only allow quick burst runs in around a minute and a half but also extensive suspension testing and the like. I have used SSR5 since GT2 for these reasons (in addition to Midfield, which fell out of use when it was butchered for GT4), and the only track I feel combines these elements the best (meaning without bumps, as I tend to ignore them anyways and don't feel they do much regardless) besides SSR5 is Grand Valley, which is a touch too long for my liking.
El Capitan also ranks up there, but I dislike it's gravel traps as opposed to guard-rails as it takes longer to get going after messing up there. The huge hill also seems to be more trouble than its worth, and the track is a little too much acceleration dependent for my taste.
Hong Kong (and for the same reason, New York and Seoul) I found useful for quick interchange testing (and also low power oversteer in muscle cars), and Fuji '80s for downforce settings adjustment, but they both lack variety (and Fuji 2005 is an atrocious track, both in game and in real life; with Fuji '90s being lame).
Suzuka is an exemplary track to test racecars on, but cars without huge downforce (ie. more than the aftermarket spoiler can provide)
suffer quite a bit there.
Autumn Hill was a track that I found favor in in GT2, but fell out of use in GT4 because I simply dislike its details, such as the ridiculously high curbs.
Infineon Sports Car Course was another one I liked initially, but I found it was only worth using in lower power (below 600 BHP) high downforce cars, as the beginning uphill chicane segments are a thing I can never get right in any game I play that features the course.
Deep Forest has good details and some very technical segments, but it is hard to see within some areas of the track where you are going.
Tusukuba has potential to be a great track for gearing purposes (SSR5 has more curve variations), but its let down by its lack of elevation.
Last but not least we have Apricot Hill, which I utterly despise in GT4 for reasons I can not place, but possibly relating to my never-ending bad memories of the track from every incarnation (GT2 through GT4). I concede it is a good track, but it is a little too acceleration biased for my liking, making lap times of slower cars bleed on longer than they do relative to fast cars in SSR5.