I've stayed at the Renaissance Fallsview, Sheraton on the Falls, Marriott Fallsview, and Hilton Fallsview. All were very nice, but it really depends on you. If you want a hotel that has a nice pool and fitness center(centre) The Renaissance and the Hilton has both. They both have great pools(The Hiltons pool has a water slide).
The Sheraton on the Falls has the best location (right on clifton hill) but does not offer any other accomdations. It has a small pool that is nice when there is less then 30 people staying at the pool. It also gets very busy during the summer months.
They all offer "Fallsview rooms", but some are better then others. The Hilton and the Renaissance are next to each other and are acorss the street from the new Casnio that is under contruction, so don't plan on getting a good view of the falls from those two. The Marriott and the Sheraton have great views of the falls, but for me its not really worth the money. If you want to look at the falls take a walk down to them. The falls are a short walk from all of these hotels. The Sheraton Fallsview room's has a small sliding door that you can open to view the falls.
Beware of the following:
Parking charges: Must of the Hotels will charge you any where from $10-$16Cdn a day. Also you will have to walk a good two blocks to get your car when staying at the Hilton, Sheraton, or Marriott. But the Renaissance Parking lot is directly behind the hotel (very easy to get to). Or you can pay $20Cdn for valet at any of them.
Hotel Food: For the price you pay at any of these Hotel's for dinner it is not worth it for what you get. Unless you get a package that includes some meals.
Summer: Very, Very, Very busy. I waited 15 minutes for the elevator to bring me to my room at the Sheraton.
Dont let any of that scare you away because Niagara Falls is a great place for a day trip or a week trip. Alot of fun stuff to do and a short ride to Buffalo or Toronto. If you really want the city to yourself go in January, you'll basically run the place and almost everything is still open.
To do List:
Maid of the Mist
Observation Tower
Great Canadian Midway
If old enough, Casnio Niagara
Hershey Store
Fireworks (in the summer months)
Journey Behing the Falls
Cave of the Wind Tour
And I'm missing alot of other stuff, but theres a start. Hope that helps