Definitely my favorite dim sum is shumai, a type of Chinese dumpling, originating from Inner Mongolia.
Unfortunately, shumai(Standard Chinese is shaomai), is not so popular as different Chinese style dumpling gyoza(potstickers) in Japan. Gyoza are served at all Chinese restaurants in Japan and also available in most of the ramen noodle diners too. However, shumai can be found only in Chinese restaurants which offers dim sum. Which means there is no Chinese restaurant which offer shumai but not gyoza in Japan.
On the other hand, deep fried shumai(ordinary shumai is steamed) can be found in pubs and taverns which do not offer gyoza at all. May be the reason is that easy to prepare than gyoza, which deep fried version is not so common in Japan.
There are many types or fillings of shumai. The most popular one is pork. Then, shrimp or crab. Utsunomiya city, gateway to Nikko is famous for gyoza but shumai city is Yokohama. Shumai bento box can be bought at Shin-Yokohama station on Tokaido Shinkansen.