I always save up my 'deposits' for any time I am nearby a luxury shopping mall* which, in hong kong, is very frequent. Definitely avoid any New Territories toilets in particularly 'local' areas, a key example being the public toilets near Yuen Long library, in which you could train a dung beetle to scuba dive on the job.
The Westrail MTR station toilets are pretty good but standards are slipping. 5 years ago they were perfect, now you need to try 2 or three cubicle doors to get the one without shit on the floor.
*when i say 'nearby' a luxury shopping mall, I should add that I actually enter the mall and find the toilets. I am not the guy shitting on the floor next to gucci.