Reagan leads by whopping margin. Look, I love Reagan, more so now from the distance of history, but George Washington is by far the most influential, simply because he led our army during the Revolution, and more importantly, established the model of the President as Chief Executive and not a king-like figure, and surrendered power when he easily could have held it for much longer, if not for life. In his time, this was a truly revolutionary idea.
Go here to cast your ballot. The Atlantic's 100 most influential Americans
(By the way, one of the great tragedies of the Iraq War was the death of Michael Kelly. Kelly was an old school, urban ethnic Democrat, and editor of The Atlantic at the time of his death. He changed it from a standard left wing rag to a thoughtful journal of opinion that highlighted the challenges of the Left and the Right. Sadly, it reverted to its previous incarnation after Kelly's death. I declined to renew my subscrition about a year after Kelly died for this reason...it's just not interesting anymore.)