There are a lot of potential candidates for the title of all time best director or best actor in Hollywood. But I think few would dispute that the all greatest composer of Film Scores is John Williams.
Between Jaws, Star Wars, Superman, Indiana Jones, Jurassic Park and Harry Potter, he's made most of the most Iconic film scores of all time. Harry Potter is the only of those franchises I haven't seen, but I still recognize the Music when I hear it.
Who are the top composers of other mediums? Back when G4 still existed and still had more then just one show about Video Games. I watched a half hour biography they did of the main Composer of the Final Fantasy series. He seems like a good candidate, but to me the most personally Iconic video game music is definitely the music form Nintendo first party titles.
But I think easily the John Williams of Anime is Yuki Kajiura.
She did Noir, my favorite Anime, largely the reason I got into anime. It's soundtrack is still the only Anime soundtrack I've ever bought on CD. And to this day it holds up, I can put that Music on my not quite an IPod and goes weeks listening to no Music other then it.
After that she's done it's sister series, Madlax and El Cazador de la Bruja. Puella Magi Madoka Magica, Fate/Zero and Sword Art Online. And many others I still haven't watched yet, some indentI intend to try soon. There is some where her Music is the only nice thing some people have to say about it, not unlike how Prequel haters feel about their Music.
Can any other Anime composer compete? Lots of Anime have memorable opening and closing themes, but nothing epic in terms of score. Death Note's score is memorable mostly for how obviously it's going for an Omen vibe, and that's fine, it works great for what Death Note was doing. And you've got lots of Anime where the characters are making Music in the show. Evangelion is an anime who's most disappointing quality was it's music, I love that Bohemian Rhapsody AMV because I feel it tells the story musically far better. The music of Fate/Stay Night: Unlimited Blade Works does as well as it does largely because it's following Fate/Zero's lead, knowing Yuki will be back for the Heaven's Feel movie has me even more excited for it, it's too bad the Dub probably won't get a theatrical release.
The only Anime who's score can compete with Yuki Kajiura scores is Revolutionary Girl Utena. And it had two composers, the Duel Themes were done by a different composer then the rest. And neither had done anything else I'm currently familiar with, partly because not much else that is considered a must watch.
So for the time being I feel there is little competition for Yuki Kajiura as the best Anime Composer.