As far as the United States goes, Los Angeles is hands down the best metropolitan area for dim sum. Eliminating Hakkasan, which has locations in Beverly Hills, Manhattan and San Francisco (though a lot of Hakkasan's dim sum has been eliminated in LA and New York due to lunch time closures), Los Angeles has the standout Sea Harbour, Elite, Lunasia and King Hua, San Francisco has Koi Palace and Yank Sing, and New York has nothing beyond Hakkasan in this category. (Red Farm is excellent but it isn't dim sum. The predecessor Chinatown Brasserie was in this class but was closed to make way for Red Farm.)
Now add two top notch dim sum contenders, both to the Los Angeles area collection. China Red in Arcadia actually opened last year, but only converted to a full dim sum lunch service recently. Indeed because of this two step path to dim sum service, China Red was off most people's radar until unmasked by Kristie Hang, the other 626 Foodette to Clarissa Wei, when she wrote her Ultimate Guide To San Gabriel Valley Dim Sum and listed China Red at the top. Immediately everybody made the mad dash to Arcadia to see what Kristie was talking about. And indeed, the dim sum at China Red was outstanding, particularly the golden lava bun, the baked bbq pork bun, the Macau egg tart, and the giant dumpling in soup. Not as good as Sea Harbour, but certainly in the next tier.
More recently Shi Hai opened in Alhambra at the site where Blue Ocean (literally) blew up some three years ago, the original fire which landed emails in my inbox mere seconds apart and garnered live helicopter coverage on the TV news and posts on Chowhound. Apparently Shi Hai was conceived with the goal of being top dim sum dog in town immediately upon opening. While they failed in that quest, it still ranks up there near China Red, Lunasia and King Hua.
In doing a city to city comparison, New York is not in the conversation since they only have the ultra pricey Hakkasan (one LA food blogger who moved to New York doesn't even bother with dim sum there and saves his dim sum stomach for trips back home). Of the other two cities, Los Angeles is far ahead of San Francisco. The main reason is that all of the top Los Angeles area dim sum restaurants serve their wares off a menu, not carts. While from historical/nostalgia point of view, many diners prefer cart dim sum, those only interested is the quality of the product unanimously go for menu dim sum for freshness. In contrast, both Koi Palace and Yank Sing serve off carts, which is indeed a tribute to them for being able to produce such a high quality cart product. A small number of Bay Area restaurants serve dim sum off the menu, with Lai Hong Lounge in Chinatown being one of the best. But in looking at the best dim sum restaurants in California, even the next lower tier of Los Angeles area restaurants such as Happy Harbor in Rowland Heights, Capital Seafood in Monrovia, Mission 261 in San Gabriel, and J Zhou in Tustin (all menu restaurants) are better than what's next best in the Bay Area after Koi Palace and Yank Sing. Likewise, menu dim sum is almost unheard of in New York, but it's not surprising that the places that don't have carts (Hakkasan, Red Egg, Dim Sum Go Go) top the local options, though the latter two rank below even the third tier of California dim sum houses.
While nothing in California matches up to the best dim sum in Vancouver (Richmond BC) or Toronto (Richmond Hill-Markham-Scarborough) it's good enough to keep me very happy and isn't that far behind the Canadian rivals such that I no longer think about planning trips to Canada.