The Canadian side of the Niagara Falls cataracts provide the most spectacular overview of the whole area. There isn't really much to see from the American side except a very close view (20 feet away) of the precipice of the American falls itself. The American side also gives access to the base of the falls if you want to get wet.
The Canadian side is mostly public park, soothing and accomodating except for parking on a really busy day. Be prepared for a long (but beautiful) walk along the chasm. The bridges from the USA to Canada are crowded and in these days after "9/11", it helps greatly to have several ID's or your passport with you.