The Horseshoe Falls are spectacular, but Niagara suffers from the context being rather less stunning and the exposure to them eventually making you somewhat inured to its beauty.
There are a lot of spectacular waterfalls in the NWT that are off most people's radar because they are mostly inaccessible.
La Ronciere Falls are certainly not of the same scale as Niagara, but placed in their surroundings,
they're stunning.