the ones that are made of Pressure treated wood and on 4x4 posts acting as skids. This keeps the unit slightly off the ground and makes for a nice flooring.
YOu should first access your wind conditions and anchor the unit down first to ensure it does not blow away. Normally screwing the unit to the decking will suffice.
Before I build my deck I make sure the ground is level and no holes to cause ponding. Then build your decking, I like about an extra foot all around the shed for extra storage. Then screw the lip of the shed down to the decking.
I have seen only gravel, I have seen bricks, and even shingles, but these all will allow water to pond in there and all sorts of new problems. A deck of Pressure treated wood and Off the ground is the best route.