Some might say FDR, some might say LBJ, others might say Nixon. The reality is that the power of the Legislative vis a vis the Executive is in constant flux.
In terms of sweeping policy initiatives FDR's administration might be the time when the Presidency took on many of its contemporary roles. The activism of the LBJ administration was a further expansion of the New Deal-era role of the FDR administration. LBJ also was arguably the first president to use the US armed forces in foreign engagements without Congress declaring war (Gulf of Tonkin resolution)--a precedent we have become all too familiar with. In terms of 'imperial pretensions' Nixon assumed all the New Deal, Great Society, civil rights activism, and the ability to intervene militarily of the preceding Presidencies and expanded them to include unfettered use of the CIA and FBI.
