I have seen posts for best burger in Manhattan but nothing for sliders and even though you could hold a good debate as to them being the same I believe they are distinctive from each other.
So to start this off, I don't know if they are the best in Manhattan but they are probably the best I have ever had and they are served at Bar Pleiades in The Surrey hotel on 76th between 5th and Madison. This is most likely in no small part due to their true origin, Cafe Boulud, located across the way in the same hotel.
Give 'em a try and report back and/or add to the list!
20 East 76th St., New York, NY 10021
20 E 76th St, New York, NY 10021