I'm pretty much onboard with everyone else in this thread.
JK Rowling takes the cake for biggest ego. She commands an army of brainwashed Twitter followers who will obey her every command. She believes that just because she made billions off a fuckin' children's book, she has some kind of say in the public sphere. I've always been a believer in the work standing apart from the author or artist, and even having a greater amount of attention to it rather than the author or artist. But Rowling has made it all about her.
GRRM? All I know about him politically is that he's rather liberal, was a conscientious objector during the Vietnam era, and made a tweet about how "Winter has come" when Trump won the election. As for ego? Shit, the only ego I get from this guy is wanting to be the next Tolkien (He even stole the R.R. ;^) ). But then again, pretty much every fantasy writer deep down in their mind wants to be the next Tolkien, or at least have close to the same degree of success. If GRRM had an ego the size of Rowling's, we would have The Winds of Winter and A Dream of Spring Part all in the same year but with shit writing, shit characters, and a far cry from what ASoIaF was. While I'm not a fan of him taking his time with the books, I'm kind of glad he isn't rushing it just to appease the rabid fanboys. If anything I think it shows he's taking great care to perfect his work, even if it means that it never sees the light of day.
Don't get me wrong though, I kinda wish he'd hurry the fuck up.