A number of historical reports say that the first European to visit the Falls was a Jesuit Priest by the name of Father Louis Hennepin in 1678, however many believe the first European visitor appeared at the Falls a number of years prior to that.
Etienne Brule arrived in Niagara in 1626. He was one of many missionaries that came to visit the Neutral Indians along the Niagara. While he was that close, it's hard to believe he wouldn't have at least seen the Falls.
Rene-Robert Chevalier and Sieur de La Salle, french explorers were known to have briefly visited the Falls in 1666. In 1678, La Salle and his party established an outpost on the Chippawa Creek where it joined the Niagara River. They spent a whole afternoon at the Falls in December of 1678. At that time Father Hennepin, drew a picture of the Falls. That same picture appears in a book published in 1699.
So, although some may believe Hennepin was the first to visit, because of the picture he drew, he was actually just the first European that had visited it, drew a picture and wrote a story about the Falls. More than likely the first to visit would have been Brule.