Yuki Kajiura definitely stands near peak in my mind. All of her works in a large portion of the Fate franchise and Black Butler has been astounding and entirely completes the atmosphere of every vibe in each scene from the series, which is difficult for me to say for really any other composer so consistently.
Though if I had to pick a contender, it'd probably be Hiroyuki Sawano. He has some pretty legendary work that I never really find myself unimpressed by. Between the Xenoblade ost and Blue Exorcist, Seraph of the End and Kabeneri of the Iron Fortress.. hard to pick.