I've had the pleasure of staying at all 3 in the last 18 months! My order woould be : Sheraton (especially if you can swing a harbourview room), then the Langham just behind and the Marco Polo a very distant third. The Sheraton only wins on the harbourview location - the Langham is a cracking hotel but the rooms are a little dark. Both have excellent Cantonese restaurants, though the Tang Court at the Langham just edges it.
The three Marco Polo hotels in Harbour City/Ocean Centre are dismal and to be avoided.
Sheraton has a Towers section which is another HKD400+ per night, but good value. Gold and Plat do not get you access through. I've stayed in one of the Langham's exec rooms but it was a while ago and I can't really remember much. Both have a small pool.