Which side of Niagara Falls should I take a boat tour on for the best rainbow photos?

Take the Canadian side Hornblower tour for the best rainbow shots. You approach the Horseshoe Falls head-on with the sun behind you in the afternoon, creating prime rainbow conditions as the mist catches the light. The US side Maid of the Mist runs parallel to the falls, giving you smaller, harder-to-frame rainbows. Go late afternoon for a big rainbow arcing across the base, and bring a rain cover for your lens.