There are just too many popular outlets... does anyone have any idea which ones have the most variety of books?
Also, can anyone recommend me guidebooks for O'level: Chemistry, physics, and SS?
I think Bras Basah complex has the most varied array of guidebooks.
For O level chemistry guidebook, try the textbook author Tan Yin Toon's "Perfect Guide - Chemistry". It is very well-written.
Usually, Longman publishers books are good. I cannot help you with other subjects.