I'm surprised nobody has mentioned Montmorency Falls near Quebec City yet.
Niagara Falls is spectacular, especially in terms of the sheer volume of water falling over the cascade, but, pound for pound, I think I prefer Montmorency Falls.
Montmorency has it all. It is actually a little taller than Niagara, and is well served by the cable car to the top, the bridge over the brink and the staircase down the cliff face to the east of the falls. Overall, it is an excellent experience!