The most frequently used material in direct restorative dental procedures is composite resin.

Composite resin has become the material of choice due to several important advantages:
- Esthetics: Composite resins can be closely matched to the natural tooth color, providing superior esthetic outcomes compared to amalgam and other materials.
- Adhesion: Composite resins bond chemically to the tooth structure, allowing for more conservative cavity preparations and better retention without extensive mechanical retention.
- Minimal Invasiveness: Because of the adhesive properties, less healthy tooth structure needs to be removed, preserving more natural tooth tissue.
- Versatility: Composite resins can be used in both anterior and posterior teeth and can be layered to improve strength and esthetics.

- While amalgam has historically been widely used due to its strength and durability, its use has declined primarily because of esthetic concerns and the availability of improved adhesive materials.
- Glass ionomer cement and resin-modified glass ionomer are useful in certain clinical situations, such as when fluoride release is desired or in non-load-bearing areas, but they lack the strength and esthetic properties to replace composite resin in most direct restorations.

Therefore, the correct answer is Composite resin.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition, Chapter 12: Direct Restorative Materials / Page 345-360