Leather is by far the most popular guitar strap material. Everyone I know who plays an acoustic owns at least one leather guitar strap. This is because most people (except for vegetarians) enjoy the look and feel of leather and it is known for its strength and long lifespan. Another big reason why leather straps sell so well is that they match perfectly with natural finish acoustic guitars. Check out the picture below for a great leather guitar strap with an awesome design.
Of course this is not the only design there is. There are countless sizes, shapes, colours and designs of guitar straps to choose from. I personally love this design
Check out the Leather Strap Designs