There is seemingly an endless list of individuals that have made significant impact not only in the enterprise but in major market sectors. To add to Semiguru's list, we could name Jack Welch (GE), Michael Dell (DELL), Bill Gates (Microsoft), John Chambers (Cisco), and so on.
I agree that one person can impact a large enterprise, but it has to be a very special person with the power to influence the organization. Jobs and Madoff are at the helm. It is much more difficult to make an impact from elsewhere in the organization. It is also much more likely to ruffle feathers when you try.
@iniewski: For one individual that can impact a large enterprise; Does possibly Steve Jobs at Apple, or on the negative outcome "Bernie" Madoff of Bernard L. Madoff Investment Securities LLC fame ring a bell for you?