I have not tried the pineapple buns from Pineapple King but have heard they are quite decent (if you are not picky and not comparing it to HK).
The addition of filling inside the pineapple bun has been due to variations and attempted innovation or modifying on a classic theme to sell more. For example, putting in BBQ pork and making it a dim sum item as popularized by the likes of Tim Ho Wan and other bandwagon jumpers.
You can find a Taiwanese version of it at Sheng Kee Bakery (sometimes they may have both the TW and the HK versions, but as it has been a long time I have not really tried it).
In San Francisco a majority of bakeries (and master bakers) are not Hong Kong Chinese, mostly southern China (e.g. Toishanese), so their execution comes out different. And then you have Sheng Kee which is Taiwanese style but in a way not close.
I don’t eat HK style baked buns much anymore. But here are my recommendations
Mr Bread as JKB has already mentioned, probably the most well balanced overall of all their offerings. I actually am not too huge on their Macanese baked egg tart, but rather their regular HK style egg tart (dan taat). Their other baked buns are decent. Their egg pie (custard like), a whole one for $7, is surprisingly great. Do try others like pizza bun or tuna.
The baked BBQ pork buns from Lung Fung Bakery are not bad and famous for yelpers, but too large and disproportionate to my liking.
A friend loves Napoleon Super Bakery (Chinatown) for their durian buns. Their other buns are a bit rough around the edges in balance, giving them more of a blue collar rustic feel, but if you are not picky these are of good size and value.
AA Bakery right across the street has some tasty savory buns (I can’t recall which ones I enjoyed). Doesn’t look the best structurally, but good enough.
Cherry Blossom Bakery (Clement Street) was fairly decent in memory but not my favorite.
On Noriega Street, Cafe Bakery and Restaurant has some decent selections and they’re one of few places that does a near proper HK style savory chicken pie (pseudo puff pastry). Beware though, grease and butter overload. There is also a puff pastry that might have some mushrooms in it with meat, or just meat. If you are not on a diet, go for it.