You said you were researching but didn't say where so I'll give the information for one on the upper west side that I have had marvelous experience with for over 3 years.I have a friend who uses Biscuits and Bath for her Yorkie and likes it a lot, though she doe use a different location (Riverside Boulevard).
I, too, am researching so I'm curious to hear what others have to say.
Here is the one closest to you.
Upper East Side
1535 First Avenue (@ 80th St)
New York, NY 10028