Vista and later VistaPro.
Prior to these, terrain rendering - particularly accurate terrain rendering with USGS DEM data was the realm of SGIs and supercomputers. Oh you could do it in TurboSilver (indeed Vista has an "output to TurboSilver" button) or any number of early 3d packages, but Vista/Pro's UI was and still is unmatched for simplicity and ease of use. Finding neat ways to travel across Mons Olympus or Crater Lake and so on was almost a game unto itself.
Even though the app grew in functionality it never grew in user complexity; many have tried to replicate what it does and how well it does it (see Bryce, VueScape and Terragen) but none of them hit that sweet spot of output quality and user friendliness that Vista/Pro does.
It's a windows app now, but if you've used the Amiga version, you know exactly where everything is and what everything does. I just wish the author would update it to include some additional features (volumetric clouds, grass and reflective/semitransparent water would be nice, along with CUDA, but that's for another discussion).
So that's my pick. Vista/Pro.
