As others have said "It depends". In warmer conditions, a hammock that has a fly to protect against rain, and built in bug netting (if bugs are a problem) can be a really great option. Unfortunately, I don't sleep well in a hammock so it's not an option I can choice. When it cold (say less than 0C/32F) you need more insulation under you then when sleeping on the ground.

So for me, the answer is either a tarp (if there are no bugs), or a tarptent (tarp with bug netting) to keep small biting creatures away.

Remember than you want a pad with with sleeping bag or quilt. The pad not only provides cushioning, but also insulation which is needed as it gets colder.

--Mark