Ok, this is probably the *only* dim sum place in Beaverton, so take the "best" title with a grain of salt - but the dim sum really is quite good. In fact, it's better than any place I could find in Portland's chinatown area.
Chinatown Restaurant serves dim sum all day. If the carts aren't rolling, just ask for the dim sum menu. They also have a regular Chinese food menu, but we've been disappointed in every non-dim sum dish we ordered. Some of it was downright bad, although I heard the salt & pepper pork chops were decent.
Prices are incredibly low, but service is often poor (surprise). The younger server (Michelle) who tries very hard, but some of the other servers can ignore you for 30 minutes.
There is MSG in virtually all of the dim sum -- but you'll find this at almost every dim sum restaurant.