A carbon atom would be the lowest complexity on this list, that is because it is a lone molecule with no other bonding between other atoms. The same forces interacting between the carbon atom would be the same for the others.
Next would be methane. Methane is a hydrocarbon composed of a single carbon atom bonded covalently with four hydrogens. The electrons within the hydrogen atoms repel each other causing them to be dispersed around the carbon atom to form a
tetrahedral shape. While more complex, the other two are continuous structures made from carbon atoms bonded to gather to form crystal structures.
Graphite is considered to be a crystal structure, just like a diamond. However, the biggest difference between them is their atomic and crystal frameworks. Diamonds have a rigid 3-dimensional structure whilst graphite is composed of 2d planes of graphite with carbon-carbon double bonds connecting the layers. Due to this, a diamond would be more complex then grpahite
