The obvious answer to this question is George R.R. Martin, whose work in the '90s laid the foundation for much of the 21st century's epic fantasy. Focusing on more recent authors and trends, I think it has to be an even split between Brandon Sanderson and Sarah J. Maas for their cultural influence. Sarah Maas' work and influence have allowed the fantasy genre to reach millions of new readers, contributing to the normalization of the genre. Sanderson has a more traditional approach to fantasy but epitomizes the modern touch of having fantastical characters experience real-world emotional conflicts. Both are enormously important to the fantasy genre's landscape in the 2020s.
