There are a few different ways to answer this question. The most popular rotary cutter is probably the Fiskars 45mm Rotary Cutter. This cutter is made with titanium blade coating that makes it three times harder than steel. It can cut through multiple layers of fabric at once, making it a great choice for quilters and crafters.
Another option is the Olfa 60mm Deluxe Rotary Cutter. This cutter has a stainless steel blade that is also coated with titanium, making it extremely durable. It can handle up to eight layers of fabric at once, making it ideal for larger projects.
For those who are looking for an even tougher rotary cutter, the Gingher 8-Inch Knife Edge Dressmaker's Shears might be the best option. These shears have blades made of high carbon steel that are hardened to Rockwell C58 levels. This makes them incredibly strong and able to handle even the heaviest fabrics.