I choose to interpret "genre giants" as "contributing foundational works to SFF," so I'm going with Le Guin, who used SFF to explore social and political issues when the genres were considered a bit of a joke. She expanded SFF's possibilities and gave it the respect it deserved. https://twitter.com/blacksailbooks/status/1305539484839223296

Many people think SFF is only about mages, space fights and all that weird "woo woo" stuff. Don't get me wrong, I love pure woo woo. But Le Guin saw its potential to explore universal experiences that expanded our idea of what kind of stories SFF was capable of telling.

There's no doubting Sanderson's contemporary popularity; it's valid to see him as a "giant" depending on your interpretation of that term. But the potential Le Guin gave SFF over a lifetime of writing is enormous.