Noting that you've just replied to a topic that's been dormant for nine and a half years, I think my original intent was to highlight tracks that used the available terrain to it's fullest, but with an emphasis on any kind of extensive off-course explorable and 'secret' areas (something that used to be quite popular in the early days).
A few years later I posted my own list (sort of) in the form of this webpage:
(click the image)
However, Wilderness by Derz is the king of such tracks. (see Phin's link)
Just look at these old TrackViewGL shots, there are no objects shown, just textured terrain:
http://mtm2.com/~trackville/minimaps/tv2.htm
Derz is a local friend of mine and I witnessed the development of his one and only track as he carved on it, bit by bit, for
years. It was made without regard for a particular course, the entire landscape was formed on an epic scale with one section naturally flowing into another, and the quilt-like texturing scheme (a brilliant bit of work) is the only kind possible on a track of this scale and variety. The terrain is completely filled, and even the borders of the world wrap seamlessly together.
If you've only seen the mapped course you haven't seen Wilderness at all. The real beauty of the track is in it's free roam capabilities, everywhere leads to everywhere else, and everywhere is a 'where'. Pick any direction and you'll be going to some where, presented with numerous paths at every turn. There are low speed goat trails meandering up cliffsides, high speed banked areas. Hilly, mountainous and even flat areas. Narrow ridges, many cliffs, crevaces, and ledges, and smooth hills, swoops and jumps. Tropical looking island areas, with bridges over water, a snowy mountain region, with bridges spanning between cliffs. Mazelike intermediate areas, little 'secret' areas and some ramps to fling one just for kicks. You could drive aimlessly, or with aim, for hours, and still not see everything.
The one course version that was published
could be considered a beta test version. I have in my posession multiple .txx files that were formed along the way, with some alternate courses on the terrain. In fact, I was supposed to refine these in subtle ways (mostly with custom models) and compile them into a multi-track pod, I also had the option to create my own course layouts on the existing terrain. The truth is that there are dozens of possible courses in the Wilderness world.
It's clearly the most fully developed terrain ever and a true work of art (check it out and you'll know my praise isn't biased).