Jet Force Gemini is an odd selection. It looks pretty decent in still shots but, as I discovered last year after buying a cart, it animates terribly. The game looks so incredibly jerky and unappealing in motion that it pretty much ruins the presentation.
My pick?
Super Mario 64
It seems silly to select a launch title by my reason is simple - it's a remarkably balanced game which makes excellent use of the hardware without trying to exceed its capabilities.
Many of other games listed in here, especially from Rare, suffer from very serious performance problems, among other things.
Mario 64 offers a mostly stable 30fps experience, smooth animation, and well considered assets and effects. While some games attempted to show more details such as complex stone patterns and the like, Mario 64 embraced the system's texture filtering with assets that are DESIGNED to be blurred. Most of the textures give the impression of smooth shaded colors rather than fine details. The select few textures that DO attempt to show detail actually manage to look rather clean as well.
It's not the most technically advanced game on the system, but it uses its resources so well.
I also think that Turok 1 holds up pretty nicely as well due to its high quality animation, similar approach to textures, beautiful water and great effects work.
Once you start trying to display more detail than the system is capable of while running at wildly inconsistent frame-rates well under 30fps things start to fall apart.