We used the Greyline company although there were other operators. I would definitely recommend this tour when visiting New York as you get to see so much of the City and surrounding areas. The guides were very informative and entertaining and knew lots about the history of the City, it’s buildings and also where all the films were made and where all the famous people live!
The Greyline buses were a bit dated and some of the sound systems were a bit iffy but overall they were very good value for money. Ours was $52 for a 2 day pass which included unlimited hop on hop off tours around the city, ferry trip to Brooklyn, and past the Statue of Liberty all the way round up the Hudson River, a trip out to the Bronx and a night time tour over Brooklyn Bridge and round Brooklyn. If it is raining you get waterproof ponchos to put on, we went in May and it was a little cool so a coat is a must on the open top part.
I would imagine it would be a little difficult for some people to get up and down the stairs but you can sit downstairs but the views/sights aren’t as good.