Leather is the only suitable material for dress belts (or a really good imitation leather if you’re vegetarian).
Cowhide. The most common leather used for belts, and comes in a few variations: full-grain leather is the best looking and most hardwearing, while calfskin is the softest and most supple. The highest quality formal belts will be full-grain calfskin.
Ostrich, lizard, crocodile, and alligator. These skins are all technically more casual belt materials, but can work as formal in a dark color.
Ostrich belts are a bold look, dotted with pockmarks from the feathers. They tend to fetch a high price.
Crocodile and alligator belts are even more upscale and have a subtler pattern and texture.
Faux leather is made with polyurethane on a fabric backing. It’s durable and low-maintenance, and the best faux leathers look realistic, although they don’t develop the same patina as real leather. Cheap fake leather looks, well, cheap and fake.