The most influential admiral depends on the causal chain being measured

Which admiral had the greatest impact on world history?

There is no context-free choice; Beatty is one plausible answer because his support for naval treaties carried far-reaching consequences.

The choice changes with the period and with how many later consequences are followed. Beatty is a defensible candidate because his acceptance of British participation in naval treaties affected the interwar order. Influence can lie in an institutional decision and its long chain of effects as readily as in one famous command at sea.