That totally depends on what you like to do in Genshin. But for more general use, Nahida's passive talent is above and beyond the best if overworld exploration and collection are your things.  If you love seeing big numbers appear when you fight bosses, Childe’s passive ability will be the best to use. But if you're hurting for crystalflies to condense your original resin, then give Sayu or YaoYao a try. They're bound to help you out!