The bones of some of his lighting algorithms are still in use to this day. I don't think it's an exaggeration to say that he is an exceptional programmer.

The best of his generation is harder to assert just because that generation had a pretty large handful of names who each truly revolutionized computing. But I do believe that without Carmack then Doom never happens, idtech never happens, the resulting 90's-2000's FPS craze never happens. Without Doom landing on the hardware of every nerd on the planet it's very likely the middle age of the video game industry went in a very different direction.

I'd say he earned that ego, even if I wouldn't like dealing with it in a professional environment. But I think it's a reasonable thing to say that a good 75% of the next decade of game dev was primarily borne up by Carmack's bootstraps.