All 3 components of the falls are quite beautiful, but the Horseshoe Falls are, in fact, Canadian.
Here is some dry, but interesting history:
The International Boundary line was established by the Paris Peace Treaty of 1783 and later in the Jay’s Treaty of 1794. The was also agreed to at the Treaty of Ghent in 1814 following the War of 1812.
The International boundary along the Niagara River was designed to follow the center line of the river between Lake Ontario and lake Erie.
If an island was in the way the path of the greatest depth of the water was followed. It was assumed that the greatest depth would be nearest the center.
Since being drawn the international boundary has not been changed. However, erosion and repair work has altered the position of the Horseshoe Falls.
The Boundary Waters Treaty of 1909 was created to resolve boundary disputes.
When they drew the international boundary taking into consideration the center line of the river and the secondary island (Goat Island), it gave most of the Horseshoe Falls to Canada. The split at the time was 90%-10%. Since then the Americans have reconstructed Terrapin Point due to erosion. It was during this reshaping of Terrapin Point that the Army Corps of Engineers expanded the land mass outward from the original Terrapin Point so that today 100% of the Horseshoe Falls is now within Canada.