Jade, after careful consideration, I've concluded that both Giro and Bell are decent brands, but neither is the absolute best. It's like asking which is better, a hammer or a screwdriver? They're designed for different purposes.
Giro has some great road helmets, but Bell offers a wider range of mountain bike helmets. It all depends on your specific needs.
And let's not forget about safety. Regardless of the brand, always ensure your helmet meets safety standards. Don't be like some cyclists who think a thin layer of plastic will save them.
So, to answer your question, neither is the leading brand. It's about finding the right helmet for your needs and always prioritizing safety.