That's one reason that these "greats" lists are so tough. "Great" doesn't necessarily mean "innovative;" "innovative" doesn't mean "influential," and "influential" doesn't necessarily mean unique or creative. Finally, none of that means that a designer was the first to do something, or even if he'she was first, that he/she did it the "best."
For example, as I said, Helmut isn't particularly influential, but then again RL isn't particularly "unique." Helmut had uniqueness to spare, but really wasn't all that innovative.
CK was innovative, influential, and the first to do many things (slim, minimal, sportswear fabrics), but he's hardly unique or the "best" at it, for I think Jil or early Prada did the "minimal" vibe pioneered by CK much better than he did.
Valentino could get out his sewing needle and run laps around RL, and with a piece of paper in front of them create 150 stellar looks in the time it took Ralph to unscrew the top off of his fountain pen. BUT, RL is assuredly more deserving of a place at the "Greatest" list, while I said above I'd not even include valentino anywhere.
Gaultier is almost assuredly more unique, innovative, and influential than Hedi Slimane, but Slimane is the reason why Gaultier's looks these days are "fresh" rather than goofy, 25 year old costume theatrics.
So, it's a very difficult discussion to have, but one that is interesting both given our own preferences and the particular criteria we'd use to define "greatest."