The Chairman Group I mentioned is around Central/Sheung Wang but might be a little bit hard to access with a taxi.
Lei Garden is definitely a good recommendation (in CWB and IFC, other branches are not as good). They serve good consistent high food quality but are not that innovative. I do like their double boiled soups.
Luk Yu is a classic and can be very good (their pork lung soup is excellent) but customer service can be hit and miss (and much more likely to be a miss if you're not a regular) and, likewise, food quality is a bit inconsistent. It can be really great but it's a bit of a lottery. Still, I think that for the environment it's an interesting place to go to. If you're more flexible on times, I'd recommend going there after 3pm on week ends as you can then sit on the ground floor that's normally reserved for regulars during rush hour. Usually, service on the ground floor is much better.
I don't love mid range restaurants from Maxim's group as they tend to skimp on ingredient quality (for example Chiu Chow garden doesn't use fresh shrimps). The square from the Maxim's group is quite good though. They are not perfect in term of techniques when making steamed dim sum so the rice cover is not as good as Lei Garden for example but the fillings are really good and use high quality ingredients.
The Hong Zhou Restaurant in Wan Chai is another good choice. It's Hangzhou food maybe slightly adapted to Cantonese tastes. The fishball soup there is amazing. Best fishball soup I've eaten. Then longjing infused shrimps are a must try in a group (portion is a bit too big for 2 people).