I've heard of Gymboree, think it's been around for a while and quite well-run. But not sure how big classes are or what costs are like.
If you live in Midlevels, you might like to consider the Playgroups at Woodlands - also situated in Midlevels, on Caine Road. Having lived & worked in HK for 10 years, I am familiar with several educational organisations and can compare a little. Woodlands offer English-language, small-group classes, morning & afternoon sessions. All their teachers are qualified, registered, experienced and hold early childhood first aid/care certificates etc.
If you'd like more info or have queries feel free to contact me on jupeter@hknet.com
I took our daughter to Gymboree from about 12-18 months and I thought it was quite good for her. It's not a "playgroup" as such, at least I wouldn't call it that. It's more of a safe gym-type environment (climbing, tunnels, etc.) with classes geared to various ages/abilities--precrawling to about the age of 4. It's geared to making motorskill development really fun. There's a structured component with songs at the beginning and the end and some bubble time and time in the middle for playtime on the equipment.
The best thing for you might be to sign up for a trial class and give it a try. If this the sort of think you are looking for, you might also try a trial class at My Gym (Centrium Building on Wyndham Street) and at Panda Junction (Lippo Center, Admiralty).
You can also look into PPA on Borret Road in the Midlevel's for an inexpensive mostly freeplay environment.