Which of these different types of solids generally has the lowest melting point

Types of solids are:
1- Ionic solids: Solids made up of positive and negative charges that are held together by electrostatic forces. Since these forces are quite strong, ionic solids have very high melting points. In their sold state, ionic solids are poor conductors. Example: NaCl
2- Molecular solids: Solids made up of molecules or atoms that are held together by hydrogen bonds, London dispersion or dipole-dipole forces. All these forces are not very strong, therefore, molecular solids have low melting points and are flexible. They are bad conductors. Example: sucrose
3- Covalent-network solids/atomic solids: Solids that are made up of covalent bonds. They are characterized by their hardness and high melting points. They are poor conductors. Example: diamonds
4- Metallic solids: Solids that are made up of metal atoms held together by metallic bonds. They have high melting points and are good conductors.

Based on this, the answer to your question is: molecular solids.