I unlocked this on Arid Sands Act 2, but I recommend Jungle Joyride Act 2 instead. Unlike many of the earlier levels there are very few hazardous jumps over bottomless pits. There's one ramp and the bit with the waterfalls, but other than that it's a very straightforward level.
On Arid Sands there's too many areas where it's easy to die thanks to the fire wizards. I died at the checkpoint before the door with the four colored keys, so there's enough points in the level after that checkpoint to get the S Rank provided you don't die after that checkpoint.