FDR, hands down. The
definitive bio was published recently by none other than Conrad "Tubby" Black. But be prepared for a long, long read.
And we pulled your bloody arse out of WW I, WW II and the collapse of the British Empire.
Must we go over this again?
By the time the US entered WWI, most of Europe had bled out; certainly, France, the UK and Germany had, and the conflict was all but over. It was the Canadians, under Borden, who broke the back of the German defences at Vimy.
Regarding WWII, the US only declared war
after a sneak attack by Japan, and never even declared war against Hitler. Der Furher declared war against the US. Go look it up.