I know you said you were going a couple of days ago, but thought I'd chime in late. I agree Prosperity's potstickers are great. I also really like the ones at XSG Dumpling House.
I don't love everything at Yun Nan Flavour Snack, but I agree that those dumplings in hot and sour soup are delicious. They're my favorite thing on the menu.
XSG Dumpling House
5301 8th Ave, Brooklyn, NY 11220