"Greatest" is a ridiculous list. "Influential" might be slightly more realistic if we try to be somewhat objective. For example, I'd put HCB in the list of 8 most influential. I have to accept that. But he probably wouldn't be anywhere on my list of greatest photographers, no matter how long the list is.

Thinking more about this, even making a list of influential photographers is not easy. Do we mean influential in some sort of complex academic way, or do we define it more simply as someone who has influenced many people to take up the hobby (and the style)? If it is the latter, I'd have to say Ansel Adams 8 times.