If we are talking about online, then probably Nakamura, but saying that he might be best ever live blitz player..? Not so sure about it. From top of my head I cannot remember him proving this at all, can someone provide me with some examples of his live blitz results or something? I wasn't following the U.S chess scene closely so maybe I missed something
From what I have seen with my own eyes though, Grischuk and Mamedyarov are absolutely sick at blitz. Anand in my opinion is the best in rapid, but not blitz somehow.