This depends on how you would measure power. If you look at which president issued the most executive orders as a metric for actions done by the presidency vice the legislature, that would be Franklin Roosevelt for sure. He issued over 3,000 E.O. according to this site: http://www.presidency.ucsb.edu/data/orders.php
However, power means different things to different people, so I would hope other folks might offer different metrics.

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